Send mail with %Mail.Message{} and Multiple recipients for email

[srivathsanmurali]

This PR implements the Mailroom.SMTP.send function that can take a %Mail.Message{} as an argument. The map is used to send all the required commands to the Mail Submission Agent.

The responses from the MSA are verfied to make sure that there are no errors. When an error response is caught, the session is closed and the EMail is not successfully sent. The send function exits with an error tuple.

When Email is sent successfully, send exits with a :ok. The same error handling is made for send_message function too.

PS. I have also implemented the escaping of the a period(.) at start of line according to rfc5321 4.5.2. I can make a seperate PR for this if that is necessary. This can be found in ce90849.