Add support for SMS notifications via Twilio

[ravron]

Hi! I kept missing the notification emails, so I hacked in support to send SMS notifications with Twilio. I didn't even pay for it – I just used Twilio's trial mode, which works fine for this application. I figure this might be useful for other people, so here's this PR. It includes:

• Add notify_sms function that tries to use TwilioRestClient to send an SMS, and fails gracefully if the twilio package is not available
• Add log level name to logging format
• Change one error print to a logging call
• Add optional parameters to config.json.example
• Add SMS documentation to README.md

Let me know if there's anything I should change, happy to respond to comments! Also let me know if you don't think this functionality is useful to enough people, and I'll abandon the PR.

[davidofwatkins]

@ravron This looks excellent, thanks! I'll merge it into dev and if others can verify that it works, I'll get that into master.

[mattmakai]

Nice! 👍