Bit of clean up StateMachine class

* StateMachine: De-nest DeferredState struct && declare under nom::priv
namespace.

* StateMachine: Rename ::size to ::count.
* StateMachine: Rename ::previous_state to ::current_state_id.
* StateMachine: Rename ::next_state to ::next_state_id.
* StateMachine: Rename class var deferred_state_ to next_state_ &&
change type from std::queue to a single struct.
* StateMachine: Rename class var states to states_.

* SDLApp: Remove (comment) out assertion for NULL state in ::states
getter.

* SDLApp: Add assertion for NULL pointer passing in
::set_state_machine.