Don't trigger both day click and drag events at once

I needed to distinguish between day click and drag events in my application.  I needed the following behavior:

* When a day is clicked, a dialog pops up with a pre-filled start time field but a blank end time field
* When a day is dragged on, a dialog pops up with a pref-filled start time field and a pre-filled end time field

This adds a configurable `mouseThreshold` option allowing clicks to be distinguished from drags.  The existing behavior can be used by defaulting `mouseThreshold` to `0` (for backwards compatibility).

There's probably a better way to do this, but this method has worked great for my needs.  Thoughts on this implementation?