Add license and homepage to gemspec.

I frequently lookup a gems homepage/repo_url RubyGems (e.g.
https://rubygems.org/gems/message_bus). These settings will be picked up
by rubygems.org upon publishing the next version.

I also added the license (MIT) to the gemspec.
Merge pull request #7 from rjackson/update_gemspec

Add license and homepage to gemspec.