Integrate Ember Data API documentation

I think it's important to have Ember Data API 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.