Update documentation

Changes include:

* Moving contributing guidelines to their own file.

* Changing the `key` and `secret` names to match the names used by
Twitter. This just reduces friction and helps those unfamiliar with
Twitter OAuth get started.
Add Rubygems' "gem version" and Code Climate's "code quality" badges

You can never have too many badges.

Also, they should all be at the top of the README for extra visibility.
Merge pull request #78 from guilhermesimoes/documentation-love

Documentation love