Split COPYING file, add missing licenses. (#1195)

Split COPYING file, add missing licenses. (#1195)

The COPYING file now only contains the modified BSD license of CVC4 and specifies the software that is either incorporated into CVC4 or can be linked against CVC4. The copyright and license information for each software can now be found in the licenses/<software>-LICENSE.

The COPYING file has now 3 sections:

(1) modified BSD license of CVC4
(2) non-GPLv3 software that is incorporated in CVC4 or that can be linked against CVC4
(3) GPLv3 software that can be optionally linked against CVC4