Add monitor API docs.

- Document different monitor types.
- Document all available options for monitors, including downtime
  and scoped muting.

**Screenshots from create API**
![screen shot 2014-10-09 at 9 45 45 am](https://cloud.githubusercontent.com/assets/472446/4577238/97ee8efa-4fba-11e4-9f4a-3c8f0a8fb411.png)

![screen shot 2014-10-09 at 9 44 59 am](https://cloud.githubusercontent.com/assets/472446/4577225/8010546c-4fba-11e4-9b7b-e489f1b448aa.png)