Supply options to 'has_many' join

Currently, `acts_as_commentable` assumes that your model is called `Comment` and provides no way of changing this. 

This pull request allows you to provide options to the `acts_as_commentable` method that will be given to the `has_many` method internally. This way, you can set any of the options that `has_many` accepts as if you were calling it directly. This does not break support for the roles either, and requires no change to existing code so should not cause any issues in version upgrades.

Whilst I didn't create this directly to solve the issue, this does in fact solve the closed issue #25.


This PR also complements PR #51, as the `gemspec` file was still referencing the `USEGA` file and not the renamed `USAGE` file, causing it to give a warning about an invalid gemspec.