Placeholder string

Hi,

I've made a small modification to allow the user to specify a placeholder string as you'd find in a UITextField. Normally one would sub lass UITextView to do this, but this isn't possible with HPGrowingTextView since HPGrowingTextView isn't itself a subclass of UITextView.

The best solution in this case is to made the appropriate changes to the internal text view subclass and handle the display logic in HPGrowingTextView itself.