Videos can now pan, vertical sliders now supported

Video panning function is rudimentary; it will be replaced with a better implementation (including zooming).

Vertical slider now supported through a global function, `sewi.createVerticalSlider()`. It accepts the following parameters, in order:

- `sliderDOMElement` - the jQuery DOM element representing an `input` of type `range`. It will not be altered in any way, only certain events will be attached to it.
- `triggerDOMElement` - the jQuery DOM element that will reveal the slider on mouseover. It will also not be altered in any other way.
- `verticalSliderClass` - an optional string representing a class or classes to be added to the generated pop-up element. This allows context-specific styling.

All further changes to the vertical slider function are to be made in the boilerplate branch.