[fix bug 1004412] Mobile nav does not close when clicking off the menu on iOS7

Seems iOS7 does not bubble up `click` events to `document`. 

* Added an additional `touchend` event to the handler. 
* Also name-spaced the events using `mobile-nav` while I was there.

Tested on iOS7 and Firefox OS and working fine, but would be good to get someone else to test on Android as well before merging.

A good page to test this on is `/firefox/desktop`, as there is no focus-able content at the top of the page.