Allow services to extend the API

I am not 100% sure this is the most efficient way or place for this to happen (we do this loop again in the services/urls.py file), so please voice your opinion!

This allows services to extend the API. There are a few things the service needs to
do:

1) Create an 'api.py' file which contains the "Resource" they are using
for the API.
2) Create a 'urls.py' (if it doesn't already have one) and add a
function to it called "register_api()". The function should look like
this:

```python
    def register_api(v1_api):
        from my_service.api import MyServiceResource
        v1_api.register(MyServiceResource())
```

When the API registers its URLs your service's API URLs will be
included.

One thing to keep in mind is that your service will most likely already
have a MongoEngine class for handling its own collection in the
database. You should use that class in your Resource. If your service
works on a specific CRITs top-level object, you might need that class.
If it works on multiple top-level objects, have fun :)