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.