GPLv3 LICENCE file added

Hi,

although SOME files have a GPL header, there is no LICENCE file in the project. I'm not clear on the specifics, but I think it can't hurt to have one (github parses it automatically, as far as I know, too).

Please check if project member and copyright information is correct, pulled it from one of the headers.

Also, should a copy of GPL be included into the project?