Add/follows

The documentation and tests are incomplete, but the functions for Follows appear to work fine. I will try to fill these out when I have more sleep, but for now I am just waiting on this to merge in order to commit some code in the new notifications web client.
We're using `add` method for this kind of endpoints.
Also keep in mind that the `query` parameter is optional

https://developer.wordpress.com/docs/api/1/post/sites/%24site/follows/new/

So the request method should be pass `query` parameter instead of `null`