Use constantize helper method from Active Support
for support nested modules, constantize can to go through all of the modules to class
Add an additional check to ensure plural resources work

In the tests, there was not one for a plural class name, a test has
been added for this.
Use module in pluralize
Feature: Allow a nil response to be parsed

It is a fairly common rails paradigm to respond with nil on a create
or update instead of the object that was modified.  Something like:

    def create
      some_model.save
      respond_with(nil)
    end

This will generate the following json: `null`

This is technically not valid JSON, as it is a single level object
however in Rapidash it will return nil instead of raising an error.
Cleanup: Make the deprecation warning more obvious

When using a plural class name for resources, indicate clearly that
it is deprecated.