Add SMTP client capability (outgoing mail auth)

This path adds support for SMTP client authentication using SASL. Sender
dependent authentication is also supported and enabled separately. More
information and examples can be found at:
http://www.postfix.org/SASL_README.html#client_sasl
Merge pull request #1 from drybjed/smtp-client

Add SMTP client capability (outgoing mail auth)