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).
was just thinking, shouldn't this rather be `_attributedPlaceholder = [[NSAttributedString alloc] initWithString:placeholderText]` so that both texts stay 100% in sync -- like Apple says for [UITextView.text](https://developer.apple.com/library/ios/documentation/uikit/reference/uitextview_class/Reference/UITextView.html#jumpTo_16)?