Added GPLv3 license to repo and header notice to all files

As agreed, the GPLv3 has been added to the main repo and a notification of copyright (or copy-*left* , as it were!) to each source file, as is the convention.

I also added the header as a text file to the doc folder so that it can be easily pasted into any new source files we write.

I named us as "A-Team Games" and our work as "ateam-tanks" as placeholders, or as the final names if we don't come up with anything else.