Allow setting resource_name 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)`.

/cc @opscode/client-eng @opscode/release-engineers 
I don't think this needs to be deprecated. I find it much nicer to user the `attr_setter` style methods from non-DSL code, and use the DSL-style getters in DSL code.