Add support for the /subscribers_count/ api endpoint

From the new api documentation released today - http://dev.justyo.co/yo/docs.html

```
Count Total Subscribers

A GET request with the "api_token" parameter.

curl "http://api.justyo.co/subscribers_count/?api_token={{api_token}}"

NOTICE THE SLASH AFTER "subscribers_count"!
```
please consider using the "request.METHOD" style here.

```coffee
request.get "#{@base}/subscribers_count/", qs: { api_token: @token }, cb
```