Various bug fixes, minor improvements, and basic skinning/theming support

In the process of installing CGP on my own server, I've made a number of minor bug fixes and improvements that are useful to me. The biggest change is adding basic skinning/theming support, as I wanted a theme (and graphs) with darker backgrounds to be easier on my eyes.

Feel free to pull in whichever of these you find useful.

Some example screenshots:

Adding load average HRULEs based on # of CPUs:

![hrule](https://cloud.githubusercontent.com/assets/2831985/2557607/5966f5da-b70f-11e3-87c7-0c549aaf50aa.png)

Another example of custom skin colors:

![mem](https://cloud.githubusercontent.com/assets/2831985/2557608/6d8b8db4-b70f-11e3-9e2a-fcb76a7ff009.png)

Graphing arbitrary data from collectd's exec plugin:

![exec](https://cloud.githubusercontent.com/assets/2831985/2557613/91c53914-b70f-11e3-9fc7-8e2a4d42e785.png)

Highlight on the ping graph when no response received:

![ping_highlight](https://cloud.githubusercontent.com/assets/2831985/2557605/3f9af85e-b70f-11e3-8f04-9344d192ae82.png)