Allow presenters to present helpers

Controllers that hide their implementation but provide helper methods
to access resources can now be used with Curly. For example, using the decent_exposure gem, instance variables that Curly relies on are no longer available to the presenter. I have extended the initializer to include a test for a helper that matches the presented name if no matching value can be found in the options or default values. If the helper exists, it is called and the return value assigned to the named presenter.