Movement Forgiveness and Custom Context

Added some movement forgiveness to the touchmove handler.  Helps with touch devices which can detect small movements when pressing hard.  Movement is based on the first finger to touch the screen.  Other fingers can still move.

Set to only triggers on left mouse button. (line 54)  I could understand if this was a bad decision on my part but since all short taps are typically a left click action, this made sense to me.  Given that, testing e.which in the callback every time seems like a waste. Might consider adding right-click / long press functionality.

Allowed an optional context to passed through to the callback for times when this would be helpful to the developer.