Adds support for general group assignment at the time of signup

Fixes #5 

- Adds support for general group assignment at the time of signup to:
```
'groupings' => array(
                0 => array(
                  'id' => 10613, // DoSomething New Members
                  'groups' => array('UserRegistration'),
                ),
```
- Stubs out doing ack after success response from MailChimp rather than in queue processing