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.
I like this name, maybe we should thinking with the complete syntax:

```js
wpcom.site('<id>').follow().unfollow(function(){ ... });
```

We could create alias:

### Follow#unfollow
### Follow#remove