* Live generation of docs is an opt-in feature. Run `ember serve --docs` to
  enable it. Generate yuidoc can be an expensive task in very big projects,
  like Ember.js itself by example, so usually when development you don't want
  to enable it unless you are specifically improving the documentation.
* Removed dependency in broccoli-yuidoc. Now it bundles its own module inside.
  The reason for that is that many code was shared between the live doc
  generation in `ember serve` and the add-hoc  generation of `ember  yuidoc`,
  particulary the code that generates the options to the YUIDoc library based
  on the `yuidoc.json` manifest.
* As consecuence of the above situation, code is much simplier.
* Also, since options are shared, there is no discrepancies in the docs
  generated by one or other approach.

TODOS:

* Make live doc generation optional