Allow users to set the +resource_name+ in an LWRP programatically

Previously this was marked as an internal API, but I am making it public. We
are reliably using this API in our LHWRPs in Release Engineering and it is
truly a delightful experience.

This deprecates using +resource_name = :foo+ in favor of the more "Chef-like"
+resource_name(:foo)+.

Conflicts:
	spec/unit/lwrp_spec.rb