HG-60 Implement internal tracking and combine namespaces

This PR implements internal tracking. Developers can add tracking code to Mercury by using the `Wikia.Utils.track(event, params)` function. I've also clarified our global namespace approach a bit. We now have:
* `Wikia` global namespace:
  * All relevant application state, modules and reusable utility functions should be collected here
  * `Wikia.Modules`:
    * All complex modules that are not simply utility functions (eg. Tracker, Sloth, etc..)
  * `Wikia.Utils`:
    * Utility functions like `extend`, `track`, etc..,
* `W` global object:
  * This is an alias for `window.Wikia`