Member index

Fixes #3603: Allows an logged in administrator to query by email or mobile via DS API.

Also adds an access argument to the `_member_resource_access` function, to restrict usage to admin's. 

Returns array of User objects that match the parameters, but only returns their uid.  The parameters query is required.

Usage:

**GET** `http://dev.dosomething.org:8888/api/v1/users.json?parameters[mobile]=2125551000`

Response:
````
[
    {
        "uid": "5"
    }
]
````

If there are no Users that match, an empty array is returned.