Add explicit MIT license declaration in gemspec

This PR makes declares the MIT license in the gemspec and adds LICENSE.txt to the list of tracked files.

For me, the license is something I look at when deciding whether or not to use a gem and being able to see it explicitly from the spec object itself also makes my little automated checker scripts work better :). Not sure how many others need this but I think its always good know what license your dealing with, even if you're just browsing on rubygems.org