Adding MIT License

If we want people using the gem for real business purposes, we'll need an open source license. Right now, the gem is copyrighted to Fastly, without specific use permissions. So, people currently using the gem aren't legally allowed to do so. Adding this license grants permission, and frees us from liability as people use the gem. See http://choosealicense.com/licenses/mit/ for more info.

This pull request adds the license in a separate file, links to the license through the readme, and adds the license to the gemspec.