styling overview chart info, added % for process cpu

First shot at some styling for the overview chart mouseovers:

![scout realtime-1](https://f.cloud.github.com/assets/7880/2033631/7d8101ae-891f-11e3-9e3a-2634edb6ab0d.jpg)

Also:

* Added "%" units for process cpu usage
* Modified `d3.linechart.js` to handle accepting metadata for multiple metrics