Fix HD/SD Toggle.

Prior to this commit when attempting to toggle between HD/SD the
Video player would remain unchanged.

The `HD` initializer included a manual click event binding, which gets
triggered ontop the existing binding provided by Video.js Button[1].

This resulted in the `onClick` function being called twice.

[1] https://github.com/videojs/video.js/blob/stable/src/js/button.js#L21
Re-write HD toggle plugin.

We wanted to use this plugin at FutureLearn, we found that:
+ The original functionality wasn't working
+ The code was coupled to the example
+ It was re-implementing some of the functionality that Video.js
components already provide.

We've refactored the code to help us understand it, and
have fixed the original issues along the way.
When toggling, resume at the previous playback time.

It's a better experience for users if the video
resumes where it left off rather than starting from
the begining as it was previously.