Refactor: Tests for StrongParameters::StrongParameters
Feature: StrongParameter unpermitted attributes

* The StrongParameters module adds an unpermitted_attrs getter/setter to
the FormObject.

* unpermitted_attrs is a hash that is populated with any unpermitted
attributes that are present in the params object.

* Added a PermittedParamsValidator that can be used to validate
whether any unpermitted attributes were identified by the
StrongParameters module. This gives uses the ability to have
validation fail if a unpermitted attributes are present.

* The UserSignup FormObject (in the dummy app) uses the
PermittedParamsValidator to validate the user attribute.

* README documentation now includes unpermitted attributes info.
Chore: Bump version to 0.5.0