feat($resource): expose promise based api via $then and $resolved

Expose $then and $resolved properties on resource action return values which
allow checking if a promise has been resolved already as well as registering
listeners at any time of the resource object life-cycle.

This commit replaces unreleased commit f3bff27460afb3be208a05959d5b84233d34b7eb
which exposed unintuitive $q api instead and didn't expose important stuff
like http headers.