Mandrill Cc/Bcc handling

Hi,

This adds the implementation of Cc/Bcc for Mandrill. There were 2 ways to do it, as exposed on the [Mandrill API doc](https://mandrillapp.com/api/docs/messages.html):

* Updating the identity to set a type, which is then parsed by Mandrill.
* Adding a `bcc_address` option

I found the first one more maintainable. Not sure how to handle tests in a proper way.
Fixed the message to be more generic.