Add option to send email via gmail or other authenticated service.

I was trying out repoguard on my local machine, and didn't have a local SMTP server to use. I wrote this code to send email via other SMTP servers (I tested on Google and Microsoft Live).
instead of prefixing, I'd categorize them under smtp key, like:
```yml
# default config
smtp:
    host: localhost
    port: 25
    use_tls: false
#    username:
#    password: 
```