Support multiple email recipients

When there are multiple recipients on an email `RCPT TO` gets sent multiple times. See section 4.1.1.3 of http://www.ietf.org/rfc/rfc2821.txt.

Support this by returning an array for `:to`

This change is not backwards compatible when there is a single recipient. I figured it would be more consistent to always return an array. I'm happy to change this if you don't think that is the best way to handle this!