Integrate Ember Data documentation

I think it's important to have Ember Data documentation available on the Ember.js website. The volatile nature of the API is all the more reason to make sure that there is a current version always available, instead of sending users to hunt through the source. It's also been a requested feature:

* Issue #343
* http://discuss.emberjs.com/t/ember-data-api-documentation/928

This commit does the following:

* Adds generating a documentation data file for Ember Data to the `generate_docs` task (it also looks for an `EMBER_DATA_PATH` env variable, just like `EMBER_PATH`)
*  Makes major changes to the structure of the `APIDoc` extension. While the extension itself is of course still a singleton and only activated once, it can now take a hash of different APIs to build pages for. *This completes much of the work towards implementing #173!* (I'd like to thank my Rubyist friends at [Hacker School](http://hackerschool.com) for helping out with this :D)
* Adds Ember Data to the options hash
* Adds a link to the Ember Data API documentation to the top bar

You can try it out by cloning the fork & running `middleman server`.

In general, this needs more polish before deployment. The documentation will need to be updated - it currently uses YUIDoc's `{{#crossLink}}` helpers, which aren't supported here. It would also be nice to support cross-linking between the two APIs. Still, I think this is a good start, and want to make sure this is the right direction to go before continuing development.