Removed unnecessary tabindex -1 from Video template.
For YouTube videos tabbing from Speeds to Volume closes Speeds dialog.

An old TODO item was done. It turns out a simple case of calling
the method to bind handlers after the Spees dialog was re-rendered.
Fixed tabbing backwards.

Now you can tab through all of the controls in Video forwards, and then tab backwards.
Adding documentation to the event handlers for volume and speed control.
Fixing spelling typos in comments.
Removed unnecessary tabindex = 0 from <a /> - they get it by default.
Optimizing code. Caching an element selector.
Merge pull request #659 from edx/valera/bugfix_tabbing_video_controls

Valera/bugfix tabbing video controls