added ability to turn off persistence. Waiting on Chris for final changes

This updates the state mixin to be aware of the use_persistence variable

Per Chris's request, this variable is invisible to the user for all blocks where it is disabled. **However** I have left it visible for the StateChange block itself, as it is not disabled by default there. **Please Comment**
I really like this idea of only putting this check inside of configure/start and not in stop because it gives the ability to 'reset' the persistence. Matt proposed that we change the name to load_from_persistence (title="Load From Persistence?").