Added an option called `dragOnMove`.

If this option is set to true, the `dragStart` event doesn't fire until there is a mousemove on the draggable element.

Looks like a lot of code but most of it is on the test page and readme.

I'd also like to look at canceling the click event if a real drag happened, but that involved some interesting timing issues so I delayed it. This part is straightforward I think. Addresses Issue [#13](https://github.com/desandro/draggabilly/issues/13).