Add files in the gem (including LICENSE).

Hi!

This lib has begun a Debian packaging process a while ago. However,since it misses license information (in the gem), it doesn't meet the Debian packaging standards.

I also added to the gem other files which could be useful:
- Rakefile to be able to run tests on a gem,
- README.md and contributors.yaml, for documentation purposes.

Could you add my changes to your master, and possibly bump a new minor version ?

Kind regards,

-- 
Guillaume