Remove validations

By default, when you use this plugin inside a model, it includes two validations: one for the presence of the password, and another for the presence of the password confirmation.

The consequence of this is that the plugin now has more responsibilities than just provide an authenticatable extension for a model.

This also makes the plugin implicitly assume that the model has the sequel validations extensions, which is not always the case.

The validation conditions (the context where the validation should be triggered), messages and presence are not configurable as well.

In my case, I have to provide password_confirmation on a simple `create` call on a console or I can't configure the way I want this model to be validated on an [administration panel](https://github.com/jeremyevans/autoforme).

So, my first approach would be to send a PR that would make the validation messages configurable, for example if you would set the password presence message to nil it would not run the validation, but that still would be cumbersome.

Another possibility is to add the validation inclusion on another file and add a notice to require this file on the README.