closes #58

In the previous implementation of the SlotMachine, I decided to put
the SlotMachine itself as an entity, but it should be seen as the real
state. It wasn't wrong conceptually, because the state is something
that evolves in time, but that design led to unnecessary
dereferencing.

This has been corrected, the SlotMachine as an entity doesn't exist
any more.
Merge pull request #60 from jeslg/state

closes #58