Add documentation generator support.

This allows us to cleanup some code in the website repo (by using the new documentation generator directly), and sets the stage for storing the documentation for a given build along with the build itself. For the documentation to be published for `ember` and `ember-data` we will need to update their `publish_build` tasks to include the documentation with the list of files to upload. I believe that this is a good first step towards true multi-version API docs.

Please note, that this does not actually change anything in the current process without modifications to the upstream project's rake tasks.