Favour eager loaded transitions.

Avoid hits on an RDMBS if the associated transitions have already been
loaded. Allows for avoidance of N+1 queries on a collection of objects
when calculating, for example, the `#current_state`, by favouring pre-
fetched data (e.g. `default_scope { eager_load :order_transitions }`).
Merge pull request #34 from figwit/master

Favour eager loaded transitions.