When loaded in a Rails environment, automatically load Arrthorizer::Rails and ControllerConcerns
Move on-load code to explicit initialization methods
Ensure that Arrthorizer::Context() is available, even if Arrthorizer::Context has not been used yet
Add default implementation for ActionController::Base#forbidden()

This method can be overridden by application developers wherever and
whenever they please.
Update stub in tests to better reflect the actual use of Contexts
Fix an oversight in Role.get where subclasses of ContextRole could not be used as arguments

Transparency is improved by allowing .get() to take multiple kinds of
parameters. Previously, it could take instances as well as role names,
but it should also accept a subclass as a parameter.
Improve parameter naming in Arrthorizer::Context()
Add an equality operator to Arrthorizer::Context

Fixes #16