Fixes # 146

Added onkeydown trap of shift-tab/tab on first and last elements
in settings to keep focus in dialog until closed.

Tested in ChromeExtension, Chrome, Safari on OS X 10.9.4 with VoiceOver.
Testing in IE 11, Firefox 31, Chrome 36 with JAWS 15 on Windows 7. 

This also fixes #147 which was closed but still had problems.