* Video Player now acting as a parent that always knowing the state of
  their children.
* Events on the sub-controls are now triggered on itself, and binded by
  the Video Player instead of triggering everything on the Video Player.
* A new helper class, SubView, has been introduced to cleanup repeat
  logic on scoped jQuery selector, render() and bind()

From https://www.pivotaltracker.com/story/show/31700967
Fixes #137

Using `.ui-slider-handle` was too broad.

Using `.ui-slider-handle` was too broad.

Using `.ui-slider-handle` was too broad.

This will leave out the student that never attempt the question and
fixes the error with histogram rendering.

There might be a chance that an error got thrown after 'contentChanged'
event was fired and stop the `toggleArrow()` method to be called. This
will make sure that the navigation will still be working even some
element in the content is failed.