Use of Retry-After HTTP header.
Allow passing the resource version number under 'resource' property.
Add 'Access origin' to the header.
Issue #180: Add access-callback per public field.
Try to decode JSON the request.
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 pull request #193 from Gizra/decode-request

Try to decode JSON the request
Issue #194: Discovery for fields by @e0ipso, @nader77 and @amitaibu.