Added information from nengo_api repository

Okay, so I've merged in the stuff from the nengo_api repository, as per the discussion in issue #12. I essentially replaced the docs folder with nengo_api's docs folder, and replaced model_object.py and object_api.py with a mix of those two files and what was in nengo_api.

Would definitely like comments on this, and whether it's preferable to go this route and flesh out what's in api.py and remove the nengo_api repository, or if that should remain just as documentation.

Also note that this is a pull request from my new add_api_docs branch to the ref-impl branch, not master. I'm mainly doing this to get feedback.