Change to GPL v3 to match Org license.

Thanks to kindahero for pointing this out.
http://www.reddit.com/r/emacs/comments/174en3/twikiorgmode_first_emacs_mode_and_looking_for/c825pr8
Update README to reflect GPLv3.
init.el not needed.

Thanks to Fuco1337 for pointing this out.
http://www.reddit.com/r/emacs/comments/174en3/twikiorgmode_first_emacs_mode_and_looking_for/c823ur8