Fix touch support for some Android and Windows 7 devices

As referenced in [mapbox.js issue #122](https://github.com/mapbox/mapbox.js/issues/122), two-finger zoom caused map to disappear due to a lack of event.scale support. 

I also fixed a quirk where the device sometimes misinterpreted the number of touch inputs on touchend which resulted in a lack of calling the onPinched function.

Thanks to @violabg for tracking down the source of the bug.