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
Update README.md
Removing debug output
Merge pull request #109 from chrismacp/master

Removing debug output
Update README.md
Fixed some problems with key navigation.
Fixed CSS problems with volume bar.
Added new API method clearMedia() to prevent browsers download media when player is destroyed or ngRoute changes. Close #112
Fixed bug when autohide is true and user moves mouse wasn't showing the controls. Close #111
Fixed bug when a source is changed but there are no tracks.
Removed unnecessary scope variable on overlay-play.
Improved docs.
Improved demo.
## v0.6.1
* Added new API method clearMedia() to prevent browsers download media when player is destroyed or ngRoute changes.
* Fixed bug when autohide is true and user moves mouse wasn't showing the controls.
* Fixed bug when a source is changed but there are no tracks.
* Removed unnecessary scope variable on overlay-play.
* Improved docs.
* Improved demo.
## v0.6.1
* Added new API method clearMedia() to prevent browsers download media when player is destroyed or ngRoute changes.
* Fixed bug when autohide is true and user moves mouse wasn't showing the controls.
* Fixed bug when a source is changed but there are no tracks.
* Removed unnecessary scope variable on overlay-play.
* Improved docs.
* Improved demo.
Updated IMA ads plugin to work with the new API.
Updated demo with Google IMA ads.
Updated IMA ads plugin to work with the new API.
Updated demo with Google IMA ads.
Updated version.
Added resize.
event-hook in case jQuery is loaded. see #116
Merge pull request #117 from stefanvonderkrone/jquery-event-fix

event-hook in case jQuery is loaded. see #116
Update README.md
reversed to original version, implementing the jquery-event-fix
added jquery-event-fix for function `onScrubBarTouchMove`