adding a generic Drag Gesture

sample usage:

faker.triggerGesture('Drag', function(){console.log('Done');},{x:100,
y:200}, {dx: 150, dy:0}, 400, 10);

This results in a horizontal drag of 150px from x:100, y:200 which takes
400ms to complete and fires a move event every 10ms.

The last two parameters duration and interval are optional and default to
duration 250ms and interval 10ms.