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).
just so i understand it correctly, the logic in `-setPlaceholder` and the new `-setAttributedPlaceholder` is to keep both placeholder properties in sync and support mixed calls - e.g. first setting an attributed placeholder and then setting the placeholder via the non-attributed setter which then copies the text attributes (or the other way around) - right?