Add more recognized filenames and better support for GPL/LGPL

COPYING, COPYING.md and COPYING.txt are now recognized (they are
very common in GPL'ed packages) and new regexes are used to identify
GPLv2, GPLv3 and LGPLv2.1 packages.
Merge pull request #5 from andrioni/master

Add more recognized filenames and better support for GPL/LGPL