ngTouch: bust mousedown events

This pull request busts `mousedown` in addition `click`.

Safari for iOS will send `mousedown` events with `click` events. `ngTouch` adds the class `ng-click-active` after both `touchstart` and `mousedown` events. Since `mousedown` comes 300ms after `touchstart`, any CSS using `.ng-click-active` will be applied twice with a 300ms delay between.

**[Safari Web Content Guide](https://developer.apple.com/library/safari/documentation/AppleApplications/Reference/SafariWebContent/HandlingEvents/HandlingEvents.html)** (See Figure 6-4)

**[Demo of bug on JSFiddle](http://jsfiddle.net/TylerEich/prkjg/26/)**