Modernized the gem, and a few other tweaks.

Now it looks like what a gem generated with `bundle gem` would look
like.

Also:
- Change requirements back to ActiveRecord 3.1, seen as the gem is not
  ready for prime time with Rails 4;
- Silence tests;
- Indicate which license this gem has.
Support AR from 3.0 onwards.
Merge pull request #1 from andremedeiros/modernize-gem

Modernized the gem, and a few other tweaks.