Monitoring

I'm adding Cloud Monitoring checks to the site as per #141, but first I'll need to write the Ansible modules to create entities, checks, alerts and so on. This is a continuation of the work done in #210.

## Remaining Work

- [x] Test the notifications against a full deployment of the site, to include:
  - [x] Changing the markup to trigger each `remote.http` alert.
  - [x] Taking down the webservers or the load balancer entirely to trigger the `remote.ping` check.
- [x] Hide the `ansible-playbook` invocation behind a `script/deploy` entry point, since the command-line args are getting crazier now.
  - [x] Make another PR to establish the `script/` directory.
- [x] Write `script/rmmon` to delete the monitoring work from `monitoring.yml`.
- [x] Use PagerDuty instead of email

/cc @rgbkrk, @ycombinator