Removed module by default, removed double include, updated documentation

This is going to appear like a very arrogant request which I promise is not the case.

Firstly I couldn't get the gem to work without wrapping `skip_attr_validation?` in a `Proc` like so, `Proc.new { skip_email_validation? }`. So my main motivation for forking was to update the documentation.

However it is entirely possible that I've not configured it possible and would appreciate your direction if this is the case.

Once forked, I noticed you were autoloading the module into the base model, something I wasn't terribly keen on (I should have separated this into a separate commit I realise). Regardless, I've updated the documentation to reflect this too.

Lastly, the `included` method appeared to be superfluous. But on reflection it might be there backwards compatibility.

Rich