Feature: Add strong_parameters class method

In rails 4 the strong_parameters gem is used as a method of protecting the
attributes that can be passed through in the creation of a model. The usual
way of doing this is defining a method on your controller however this is not
the preferred location for defining the strong_parameters when using
form_objects. To prevent having to define a boilerplate `x_params` method on
every form object a `strong_parameters` class method has been added.