Stubbing MailChimp methods

Fixes #10

- Adds `MB_MailChimp` class to support Message Broker interaction with the MailChimp service for member email management.
  - `submitBatchToMailChimp($composedBatch = array(), $mbDeliveryTags = array())`
  - `submitToMailChimp($email = '', $mbDeliveryTag = array())`