Add SmtpUsername and SmtpPassword fields to conf to allow authenticated SMTP

Authentication will only be attempted if TLS handshake was successful and a username or password was specified.

Fixes #417 
When I leave the split out I get:

```
2014/11/24 14:13:06 failed to send alert cpu.lookup{host=maxwell,remote=127.0.0.1} to [jeremy@transcriptic.com] use of closed network connection
```