Add alias to Audit class and fix README

Hi,

This pull request aims at fixing the following issues:

- #101 - Add alias for the Audit class, otherwise one would have to use `Audited::Adapters::ActiveRecord::Audit`.

- #100 - I had exactly the same issue. I fixed the docs in order to reflect the latest released gem.

Do you agree with these changes?
@carvil Wouldn't you just do:
::Audit = Audited.audit_class after the line above?

We've done the same but just do it in an initializer. I would like this in the main repo too