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+518@dosomething.org",
"password":"password",
"birthdate":"1995-12-31",
"first_name":"Aaroni",
"mobile":"2121231413",
"user_registration_source":"sms"
}
````