remove user data that shouldn't have been committed
rename RCSStatechart to RCSPushdownState and make its API closer to RCSState (as it really implements a pushdown automata, not a statechart)
rename RCSStatechart.[h|m] RCSPushdownState.[h|m]
made -stateContextDidEnterErrorState @optional, removed the underscore prefix, and removed -shouldTextContextDidEnterErrorState
updated the README