copy, paste, etc cannot be overridden by subclassing

[eanagel]

When using a standard UITextView it is possible to override the built-in copy/paste functions with your own functionality. This is useful for more advanced copy/paste scenarios (we have the need to paste images into a UITextView.) This kind of thing is currently not possible with HPGrowingTextView because that would require overriding the functionality in HPTextViewInternal.