Add 'Send test email' button

[cweagans]

Contains the changes for the test email button from @djvaporize 's master branch. I did a bit of cleanup, but the commit is still credited to him.

The changes that I made:

• Move the test email button up underneath the SMTP information because a) that's where it is in most other UIs and consistency is good, and b) The thing that users actually care about testing is that they have set up their SMTP creds properly.
• Reduce code duplication in init.py by moving the email sending logic to one method and calling it from the on_event() method and the on_api_command() method.

Sorry if there are any obvious errors here - I don't usually work on Python stuff, so I'm a bit rusty.

Hope this is helpful!

Fixes #7

[djvaporize]

Thanks @cweagans !! I've been meaning to clean up the code to refactor to a separate mail function but haven't had the time. Hopefully @anoved can approve this soon :).