Only inject inputAccessoryView if textView is editable.

There's a bug in UIKit that would fire a `UIKeyboard(Will|Did)ShowNotification` when a `UITextView` is readonly (`textView.editable = NO;`) but has a `inputAccessoryView`. See also: http://openradar.appspot.com/radar?id=5306932189462528

To work around this issue, `DAKeyboardControl` should only inject a `inputAccessoryView` if the `UITextView` is actually `editable`.