Add license

Currently the it's not clear which license the project is under. If your setup.py is correct I've assumed it's under the GPLv3. I've added license headers to all the files and the COPYING file (containing the full plain-text license) as well as adding it to the readme so it should be clear to anyone coming to this project which license overkill is under.

Note: I didn't know if you just wanted it under the GPLv3 or the GPLv3 (or later) so I've opted for the later however you may wish to change that if that's not the case prior to merging it in.