Only show Toolbar if ZSSRichTextEditor is First Responder

(!) not yet ready to merge

This fixes a part of #65.

**Use Case**: A view with several `UITextField`s + `ZSSRichTextEditor` in a container view.

**Fix Idea:** In `keyboardWillShowOrHide:` check if `ZSSRichTextEditor` really is the first responder. If not, do not show the toolbar.

**What Works:** If the keyboard is not shown and a `UITextField` becomes active, the toolbar is not shown. When `ZSSRichTextEditor` becomes active the toolbar shows up.

**Open Problems:** When `ZSSRichTextEditor` is active and the keyboard is visible, and the `UITextField` becomes active, the toolbar stays visible. This is due to the keyboard notifications not getting triggered when the `UITextField` becomes active, as the keyboard is already showing, and the `UITextField` does not have an `inputAccessoryView`.

Any ideas how to fix this completely?