More features for the Sequel mock
Association proxy for single objects
Populate test MockSequel by inserting

with a view to testing against a real database
Belongs to association proxy tweak
MockSequel counts reads, writes, deletes
Use proxy for single object "belongs to" associations

* Prevents potential circular loading
* Prevents many unnecessary database reads
* Removes a special case, all associated objects
  are now proxied
Ordering for has many associations
Assocations queryable via #where

Need to figure out how to test datastore optimisation vs iterating over rows