Pass `apiPath` to client-side, so API calls go to right place

Now, simply passing `apiPath` option to `rendr.createServer()` is all it
takes to for models and collections to fetch API calls from the correct
path. Before, setting `apiPath` would set up the middleware at the
correct path, but it wouldn't force models and collections from fetching
from said path.
Fixing tests.
Merge pull request #139 from airbnb/api-path-in-model

Pass `apiPath` to client-side, so API calls go to right place