Support for UITextView's with attributedText and adds attributedPlaceholders

-- Adds attributedPlaceholder property so that the placeholder may be matched to the main text view in the case that it uses UITextView's attributedText property for mixed formatting.
-- Fixes issue with placeholder incorrectly being visible after setting attributedText on main text view by observing attributedText property on the main text view.
-- Toggles placeholder visibility on becomeFirstResponder (catches the previous bug and any additional edge cases).
Yeah, I think I may have misinterpreted when I looked at the [attributedText](http://developer.apple.com/library/ios/documentation/uikit/reference/uitextview_class/Reference/UITextView.html#//apple_ref/occ/instp/UITextView/attributedText) docs. They refer to taking the attributes at index 0 and copying them, but I think you're right, we should just let text view handle that.