Add license to gemspec, is MIT

Nice project, I just saw this on the [YUI Blog](http://www.yuiblog.com/blog/2013/08/)

Including a license in your gemspec is an easy way for rubygems.org and other tools to check how your gem is licensed.  As you can image, scanning your repository for a LICENSE file or parsing the README, and then attempting to identify the license or licenses is much more difficult and more error prone. So, even for projects that already specify a license, including a license in your gemspec is a good practice. See, for example, how [rubygems.org uses the gemspec to  display the rails gem license](https://rubygems.org/gems/rails).

See my [github repo for info about my gem license project](https://github.com/bf4/gemproject/issues/1) or my [blog post about this project for more information](http://www.benjaminfleischer.com/2013/07/12/make-the-world-a-better-place-put-a-license-in-your-gemspec/).