Set User field_mobile value in DS API

Adds `mobile` as a possible property to set when using the `users` POST resource https://github.com/dosomething/dosomething/wiki/api#create-a-user

Sample POST data:
````
{
"email":"aschachter+520@dosomething.org",
"password":"password",
"birthdate":"1995-12-31",
"first_name":"Aaroni",
"mobile":"2128902342",
"user_registration_source":"sms"
}
````

Response:
````
{
    "mail": "aschachter+520@dosomething.org",
    "name": "aschachter+520@dosomething.org",
    "status": 1,
    "created": 1418163045,
    "pass": "$S$DNXN0a0dJjwllZwiGVv5lTurISIAZnBf5a85h.FA5jVYElGc8ZfV",
    "field_birthdate": {
        "und": [
            {
                "value": "1995-12-31",
                "timezone": "UTC",
                "timezone_db": "UTC",
                "date_type": "datetime"
            }
        ]
    },
    "field_first_name": {
        "und": [
            {
                "value": "Aaroni"
            }
        ]
    },
    "field_mobile": {
        "und": [
            {
                "value": "2128902342"
            }
        ]
    },
    "field_user_registration_source": {
        "und": [
            {
                "value": "sms"
            }
        ]
    },
    "uuid": "e3e298a5-fa01-422a-99c5-1793d18dd72e",
    "uid": "1702964",
    "theme": "",
    "signature": "",
    "access": 0,
    "login": 0,
    "language": "",
    "picture": 0,
    "init": "",
    "roles": {
        "2": "authenticated user"
    },
    "field_under_thirteen": {
        "und": [
            {
                "value": 0
            }
        ]
    },
    "field_address": {
        "und": [
            {
                "element_key": "user|user|field_address|und|0",
                "thoroughfare": "",
                "premise": "",
                "locality": "",
                "administrative_area": "",
                "postal_code": "",
                "country": "US"
            }
        ]
    },
    "field_opt_in_email": {
        "und": [
            {
                "value": 1
            }
        ]
    },
    "field_opt_in_sms": {
        "und": [
            {
                "value": 0
            }
        ]
    }
}
````