add addPeriodicTimeObserverForInterval to HysteriaPlayer, it will direct...

add addPeriodicTimeObserverForInterval to HysteriaPlayer, it will directly call AVQueuePlayer same method, so it just bring addPeriodicTimeObserverForInterval method to HysteriaPlayer class, also add a current time label to display current time in view controller in example. 

![ios simulator screen shot apr 13 2014 9 36 47 pm](https://cloud.githubusercontent.com/assets/782469/2691829/695fc31e-c38e-11e3-9b1f-5123b34913b7.png)