Support onenterstate and onafterevent handlers

Before, only the <strong>on<em>state</em></strong> and <strong>on<em>event</em></strong> event handler callbacks were supported. Now <strong>onenter<em>state</em></strong> and <strong>onafter<em>event</em></strong> can be used, too. They take precedence over the shorthand versions if both are present (like in [jakesgordon/javascript-state-machine](https://github.com/jakesgordon/javascript-state-machine)).

As before, <strong>onleave<em>state</em></strong> and <strong>onbefore<em>event</em></strong> can be used to cancel an event when it occurs.