MouseTracker Multi-Touch Update

Fixes/Addresses #29, #59, #357 

[Try a Live Demo Here](http://msalsbery.github.io/openseadragontester/index.html)

Added support for the [WC3 Pointer Events Spec](http://www.w3.org/TR/pointerevents/) used by IE10+/Windows 8+, and also under development for Firefox.

Multi-Touch enabled. Now supports multiple simultaneous pointing devices and multiple touch points.

Added a pinchHandler callback to MouseTracker and corresponding pinch zoom handling in Viewer. Pinch gestures are now handled without mimicking scroll wheel events.

Added a dragEndHandler callback to MouseTracker and corresponding kinetic panning for touch device input in Viewer.

Added real-time velocity (speed and direction) tracking to drag-related gestures. The values are included in the event parameters of the corresponding MouseTracker event callbacks. Currently used for kinetic panning (flick gesture) on touch devices. Can be used to detect a swipe gesture as well.