README and LICENSE tweaks

Besides correcting some stuff here and there in the README and doing some overall cleanup, I've also added instructions for setting up Misago using Vagrant as per #135. The GNU GPL license has also been updated to a Markdown version to improve readability and make use of Github's excellent Markdown parsing.

P.S.: You may want to merge these commits as well as the commit from #135 into `develop`. There shouldn't be any conflicts so it should be pretty straight forward. Just a recommendation!