Added support for up/down arrow keys on timepicker

Added the ability for the user to adjust hours and minutes for the timepicker by using the up/down keyboard arrow keys. This allow the user to adjust time without the need to use the mouse as they can simply tab across the input boxes (as I haven't used `e.preventDefault()` for the `keydown` event).
By default the arrow keys are supported but this can be turned off by using an additional timepicker attribute `arrow-keys="false"`