Added and extends function for external subclassing

[heff]

Addresses #2035

It uses the babel _inherits under the hood, with the added benefit of defining additional methods. The external API will look like the following:

 var Button = videojs.getComponent('Button');

 var MyButton = videojs.extends(Button, {
   constructor: function(player, options) {
     Button.call(this, player, options);
   },

  onClick: function() {
    // doSomething
  }
});

[gkatsev]

we should import everything at the top of the file.

[heff]

Updated. Everything else look ok?

[gkatsev]

LGTM