Introduce on_state_change for building persistent states.

I am trying to migrate from [AASM](https://github.com/aasm/aasm) to
statemachine, and I need someway to make statemachine update the
state in the database, i.e. the aasm_state column.

At first I tried on_entry callback, but it's tedious to apply to
every states. Using on_entry on superstate didn't seem to fit, either.
Introducing this callback might be much easier to do this...

Thank you for considering!
Merge pull request #13 from godfat/on_state_change

Introduce on_state_change for building persistent states.