Added InterestGroup methods

Support for these methods:
1) http://apidocs.mailchimp.com/api/2.0/lists/interest-group-add.php
2) http://apidocs.mailchimp.com/api/2.0/lists/interest-group-del.php
3) http://apidocs.mailchimp.com/api/2.0/lists/interest-group-update.php