Remove inclusion of validations by default

By default, when you used this plugin inside a model, it would include
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 had more responsibilities
than just provide an authenticatable extension for a model.

This would also make the plugin implicitly assume that the model has the
sequel validation extensions, which is not always the case.

The validation conditions (the context where the validation should be
triggered), messages and presence were not configurable as well.