support embedding of an Entity

This is really for MongoDB.
Also support the ObjectId type via Objectid
fix tests for SQL
run Entity tests against SQL
Revert "run Entity tests against SQL"

This reverts commit 61f0e4e213e0e46322e3479d6106162b18dc2602.

The tests have a weird GHC error.
Merge pull request #144 from yesodweb/embedded-entitites

support embedding of an Entity