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).
minor note: I think we should make it possible in the future to use empty usernames/passwords here ([see](https://code.djangoproject.com/ticket/15750)) therefore a ```is not None``` check would be better. Also in the config file the default username and password should be ```null``` instead of an empty string.