COPYRIGHT: Specifiy the version of the GPL

All code in pandoc licensed under the GPL version 2 or later is just
marked as being licensed under the GPL.  There are multiple versions of
the GPL, most notably version 2, version 3 and the Affero GPL.  As to
avoid possible confusion, licensing info is updated to be more specific
about the GPL and its version.
COPYRIGHT: Remove outdated copyright statements

Some files are no longer distributed with pandoc, the respective
copyright and licensing information is removed.
README: Update variable defaults documentation

The documented default values of variables now agree with their true
default values as defined in Writer/HTML.hs.
Merge pull request #1268 from tarleb/copyright-housekeeping

Copyright housekeeping