Add ability to customize the Copy/Paste menu by subclassing HPGrowingTextView

This pull request forwards on copy/paste related methods from the InternalTextView to the parent (HPGrowingTextView). This allows adding custom copy/paste functionality by subclassing HPGrowingTextView. The following methods may be overridden: canPerformAction:withSender:, copy:, cut:, paste:, select:, and selectAll:. While in any of these methods it is safe to call the internalTextView version of the method to get the default implementation.