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).
According to the [docs](https://docs.python.org/2/library/smtplib.html#smtplib.SMTP.sendmail), sendmail does not close the connection upon an exception, so this should go to a finally close. But :thumbsup: for the exception handling!