Copyright.txt: Add notice of copyright by contributors

We do not require copyright assignment for contributions to CMake.
Update the top-level notice to mention "Contributors" explicitly.
Add a list of the Contributors whose names already appear in
source-level copyright notices.  Find the list with the command:

    $ git grep -h 'Copyright[^.]' -- *.* \
        Auxiliary Modules Packaging Source Templates Tests \
      | grep -v Kitware | sed 's/^#\? *//' | sort | uniq

Manually refine the list to de-duplicate entries and filter out
notices from third-party code.

Copyright.txt

@@ -1,6 +1,5 @@
 CMake - Cross Platform Makefile Generator
-Copyright 2000-2016 Kitware, Inc.
-Copyright 2000-2011 Insight Software Consortium
+Copyright 2000-2016 Kitware, Inc. and Contributors
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
@@ -14,10 +13,9 @@ are met:
   notice, this list of conditions and the following disclaimer in the
   documentation and/or other materials provided with the distribution.
 
-* Neither the names of Kitware, Inc., the Insight Software Consortium,
-  nor the names of their contributors may be used to endorse or promote
-  products derived from this software without specific prior written
-  permission.
+* Neither the name of Kitware, Inc. nor the names of Contributors
+  may be used to endorse or promote products derived from this
+  software without specific prior written permission.
 
 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
@@ -33,12 +31,77 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 ------------------------------------------------------------------------------
 
+The following individuals and institutions are among the Contributors:
+
+* Aaron C. Meadows <[email protected]>
+* Aleksey Avdeev <[email protected]>
+* Alexander Neundorf <[email protected]>
+* Alexander Smorkalov <[email protected]>
+* Alex Turbov <[email protected]>
+* Andreas Pakulat <[email protected]>
+* Andreas Schneider <[email protected]>
+* André Rigland Brodtkorb <[email protected]>
+* Axel Huebl, Helmholtz-Zentrum Dresden - Rossendorf
+* Benjamin Eikel
+* Bjoern Ricks <[email protected]>
+* Brad Hards <[email protected]>
+* Christopher Harvey
+* Christoph Grüninger <[email protected]>
+* Clement Creusot <[email protected]>
+* Daniel Blezek <[email protected]>
+* Daniel Pfeifer <[email protected]>
+* Enrico Scholz <[email protected]>
+* Eran Ifrah <[email protected]>
+* Esben Mose Hansen, Ange Optimization ApS
+* Geoffrey Viola <[email protected]>
+* Gregor Jasny
+* Helio Chissini de Castro <[email protected]>
+* Ilya Lavrenov <[email protected]>
+* Insight Software Consortium <insightsoftwareconsortium.org>
+* Jan Woetzel
+* Kelly Thompson <[email protected]>
+* Konstantin Podsvirov <[email protected]>
+* Mario Bensi <[email protected]>
+* Mathieu Malaterre <[email protected]>
+* Matthaeus G. Chajdas
+* Matthias Kretz <[email protected]>
+* Matthias Maennich <[email protected]>
+* Miguel A. Figueroa-Villanueva
+* Mike Jackson
+* Mike McQuaid <[email protected]>
+* Nicolas Bock <[email protected]>
+* Nicolas Despres <[email protected]>
+* Nikita Krupen'ko <[email protected]>
+* OpenGamma Ltd. <opengamma.com>
+* Per Øyvind Karlsen <[email protected]>
+* Peter Collingbourne <[email protected]>
+* Petr Gotthard <[email protected]>
+* Philip Lowman <[email protected]>
+* Philippe Proulx <[email protected]>
+* Raffi Enficiaud, Max Planck Society
+* Raumfeld <raumfeld.com>
+* Roger Leigh <[email protected]>
+* Rolf Eike Beer <[email protected]>
+* Roman Donchenko <[email protected]>
+* Roman Kharitonov <[email protected]>
+* Ruslan Baratov
+* Sebastian Holtermann <[email protected]>
+* Stephen Kelly <[email protected]>
+* Sylvain Joubert <[email protected]>
+* Thomas Sondergaard <[email protected]>
+* Tobias Hunger <[email protected]>
+* Todd Gamblin <[email protected]>
+* Tristan Carel
+* University of Dundee
+* Vadim Zhukov
+* Will Dicharry <[email protected]>
+
+See version control history for details of individual contributions.
+
 The above copyright and license notice applies to distributions of
-CMake in source and binary form.  Some source files contain additional
-notices of original copyright by their contributors; see each source
-for details.  Third-party software packages supplied with CMake under
-compatible licenses provide their own copyright notices documented in
-corresponding subdirectories.
+CMake in source and binary form.  Third-party software packages supplied
+with CMake under compatible licenses provide their own copyright notices
+documented in corresponding subdirectories or source files.
 
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