Fixed Toolbar Positioning

Fixed toolbar positioning when the bottom of the ZSSRichTextEditor view is not the bottom of the screen.
This fixes 1 of the 2 issues in #65.

For an error reproduction see https://github.com/fabb/ZSSRichTextEditor/tree/vertical_position_bug
The branch `vertical_position_bug` shows a non-translucent tabbar at the bottom. The toolbar is positioned too high when the keyboard appears.