Added license to gemspec

Some companies [will only use gems with a certain license](https://github.com/rubygems/rubygems.org/issues/363#issuecomment-5079786).
The canonical and easy way to check is [via the gemspec](http://docs.rubygems.org/read/chapter/20#license)
via e.g. 

    spec.license = 'MIT'
    # or
    spec.licenses = ['MIT', 'GPL-2']