Add handle*Event options to ol.interaction.Pointer

More specifically: handleDownEvent, handleDragEvent, handleMoveEvent, and handleUpEvent.
Make ol.interaction.Pointer call handleMoveEvent
Make shouldStopEvent default to the identity func
Add ol.Map#getTargetElement

ol.Map#getTarget returns an element or a string, depending on what was passed to setTarget. This commit adds a getTargetElement method that always returns an Element.
Add ol.geom.flat.transform.translate
Add ol.geom.translate
Add a drag-feature example

This example also demonstrates how to implement a custom interaction that inherits from ol.interaction.Pointer.