DS API: Get Member Count

Closes https://jira.dosomething.org/browse/DS-444

Provides an endpoint to get the Member Count.  Must use POST, as its a custom Services resource action.  Will update wiki upon merge.

**POST** http://dev.dosomething.org:8888/api/v1/users/get_member_count.json

Result:
````
    {
       "formatted": "2,900,343",
       "readable": "2.9 million"
    }
````