Fix #86: Can click thru NavBar arrows onto commands

When clicking rapidly on a NavBar arrow, you can sometimes click thru the
button and onto the NavBar command behind it, invoking it.

This is because a NavBar command receives a z-index of 1 when it is
focused which causes it to be above the arrow. The fix is to give NavBar
arrows a z-index of 2 so that they're always on top of the commands.
Fix #236: Hub's home/end keys only work in IE

Due to WinJS's use of IE's -ms-scroll-translation:vertical-to-horizontal,
Hub's support for the home and end keys in IE is provided for free by the
browser.

To get Hub's home and end keys to work in other browsers, they have to be
handled explicitly.