Trigger ready event only after full instansiation (as documented)

I've noticed the the "chosen:ready" event is fired by the "set_up_html" method. But after this method, the "register_observers" method is called - and only after that - the Chosen is really ready. 
I stumbled upon this problem when I tried to fire my own chosen event ("open") after listening to the "ready" event - and it didn't work, because the observers were not registered yet.
This PR is my suggestion - triggering only at the end of the constructor.