Change parameter sanitizer instance method to scope to devise

This way it's very explicit that this method is for devise and it won't
run into any naming collisions with user code.
Introduce BaseSanitizer null sanitizer and controller-specific callbacks

This updates Devise's StrongParameter support to feature:

- A Null base sanitizer to support existing Rails 3.x installations that
  don't want to use StrongParameters yet
- A new, simpler API for ParameterSanitizer: #permit, #permit!, and #forbid
- Overrideable callbacks on a controller-basis, e.g. #create_sessions_params
  for passing the current scope's parameters through StrongParameters and
  a helper method, whitelisted_params, for rolling your own implementations
  of #create_x_params in your own controllers.
- Lots of tests!