Add Mandrill

This PR is the initial stage of #2312

Not ready yet.

Example inside simplate:
```python
message = {
 'from_email': 'luke@gittip.com',
 'from_name': 'Luke Strickland',
 'html': '<p>Hello world</p>',
 'subject': 'Mandrill Test',
 'tags': ['password-resets'],
 'text': 'Hello',
 'to': [{'email': 'luke@axxim.net',
         'name': 'Luke Strickland',
         'type': 'to'}]
}

result = website.mail.messages.send(message=message, async=False)
```