added `sqlQuery()` method to query module to allow executing raw SQL queries against the store's database.

Renamed query module method `select()` to `query()` for consistency reasons.
PartitionedCollection is now functional again (was broken due to recent query changes)

Moved toString methods out of prototype definition because that lead to a (hidden) exception when setting up the PartitionedCollection prototype (referencing the Collection.prototype immediately invoked its toString method causing an exception)