Abstract data providers, and add example.

Separated the RESTful handler from the underlying data provider (e.g. query builder). We now provide an EFQ data provider, and a DB data provider.
This opens the door for extending RESTful to act as a proxy to other providers (e.g. Solr, other web services, etc').

Also, added an example resource for the variables table, to show how to leverage the new data providers.
Merge branch '7.x-1.x' into 194-field-definition-discovery
Issue #194: Fix Test
Issue #194: Label changes.