* Session serialize/deserialize has an optional parameter 'from_regis…

…try'. It set, the FSM serialization will serialize

FSM's name instead of the automata definition, and deserialization, will get the automata fro the registry instead the
embedded definition.
* Ended session's don't have their stack emptied. It is necessary to get end state's name when `current_state_name`
is called.
* Finished session don't remove `state` info. Necessary to make `currentStateName` and `currentState` to give information
about the last state before getting session finished.