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).
Yep, basically textviews are supposed to have two versions of their text, an optional attributed version and a plain text version. So we're just supporting both those types of placeholders as well. And we can call either setter and preserve formatting as much as possible.