Introducing threshold lines in nagios graphs

In our nagios graph now we're having a yellow line for our "warning" thresholds and a red one for "critical", so you know graphically how far from the thresholds you are. And it's being pretty useful :)

Example:
![nagios-graph-with-thresholds](https://f.cloud.github.com/assets/5192538/2339463/14f17a94-a4b6-11e3-9de3-158c22b3b103.png)