@DocumentRef (for Ref<?> fields) and @DocumentEmbed (for embedded fields)

Introducing dependency on Objectify.
* Doco can now translate Ref<?> fields using @DocumentRef annotation.
  * All the fields of Ref<?> class are prefixed with simple name of Ref<?> type.
  * Added tests for same which shows that arbitrary nesting is possible.
  * There is a possibility of creating circular references in which processing @DocumentRef will run into infinite recursion.

* Supporting embedded fields using @DocumentEmbed annotation.
  * This is useful when an entity contains other classes which has fields needing indexing.