It turns out that Twilio is quite expensive, more than I willing to pay
for this service. So I'm going with MessageBird.

AWS' SMS delivery has been unreliable to say the least. Replace it with
Twilio but that's more complex:

- A new Lambda function to send SNS messages as SMS messages with
  Twilio.
- Add the function as a subscription to the SNS topic.

Docstrings for the different checks.

Also, with the newer Pylint version, address some new issues.

- Check base class, all checks derive from it and have a common calling
convention.
- Add retries.
- Add a DNS zone transfer check. Turns out if fails on ns1. WIP.
- A wrapper around checking web sites. Just provide a list of URLs.

Outbound traffic on port 25 is restricted by AWS. Removing the
restriction will raise the cost of the project.

Ease debugging.

Start at the basics, DNS.