Support a variance for basic onTap detection

[danshultz]

Some devices are not tolerant of fat finger touches and will issue a move event in between a down/start and up/end event. This causes "onTap" to not perform well in some scenarios.

Supporting a tolerance for what constitutes as a "tap" provides the a fill gap for devices that issue erroneous events. This was mostly observed on the surface tablet and some instances of windows 8 touch devices.

The threshold can be adjusted by changing Gala.onTap.maxContactTime and Gala.onTap.maxContactDistance.

Upon further testing on different devices this is working well without issue. I know we discussed adding an onDrag abstraction but in reviewing different functionality across books I am not sure if this is necessary at this moment in time.

Please review this pull request and merge

[danshultz]

Pull request has been updated to incorporate the feedback and remove the "max" time to be considered a "tap"

[joseph]

Thanks.