Document new monitor and downtime features

- Add tag filter for get_all_monitors
- Add downtime messages
- Add update downtime method

![screen shot 2014-12-10 at 5 04 25 pm](https://cloud.githubusercontent.com/assets/3514765/5385228/c14bb386-808e-11e4-9f09-113539e59297.png)


![screen shot 2014-12-10 at 5 05 13 pm](https://cloud.githubusercontent.com/assets/3514765/5385229/c52a9076-808e-11e4-8b0e-c3365aaafa0b.png)