Make control buttons <button>s instead of <div>s

<button>s have better keyboard accessibility, as they can be focussed
with the tab key and pressed with the space bar.
Make vg-scrubbar keyboard- and ARIA-friendly

The scrub bar can now be focussed with the tab key, and its value can be
changed with the arrow keys. It also specifies ARIA attributes
identifying it as a slider.
Make volume bar keyboard-friendly

Focussing the mute button and pressing UP or DOWN adjusts the volume by
5%.
Merge pull request #108 from soton-ecs-2014-gdp-12/accessibility

Make Videogular controls more accessible