Add the option for a zoom handler callback
Rename drawHandler to onDraw. Also make it a notification style callback
(does not override the draw functionality).
Add an overlay option 'useTransform' that will transform the overlay
element instead of moving it's postion - useful for SVG elements
issue:
    canvas pans when tap on touch screen

reason:
    onTouchStart calls onMouseDown which registers 'mousemove' event handler.
    when I tap on touch screen, even though browser did not dispatch 'touchmove' event, it dispatched 'mousemove' event.
    therefore, the 'mousemove' handler pans canvas.

fix:
    stop onTouchStart handler registering 'mousemove' handler as onMouseMove will be properly triggered by onTouchMove
Merge branch 'master' into custom_zoom_handler
fix: remove the useTransform/SVG stuff and move back to a simple onDraw callback only so people can handle their own custom drawing and sizing
Merge pull request #168 from ethanli83/fixtouch

canvas pans when tap on touch screen
Changelog for #168
Use the correct onDraw in drawer.js. Remove imageFullSize and fix up the comment for onDraw
Merge pull request #160 from lukemurray/custom_zoom_handler

Add the option for a zoom handler callback
Changelog for #160