Full support of Matrix projects and support of variables in a few fields

Hi there,

just submitting a bit of work I've done to adapt this plugin to my need.
The main point was the support of Matrix projects as well as support of variables in the Title field (due to the multi configuration support)
The end result is that for each configuration of a matrix project you'll get the relevant charts you have configured, and the root matrix project page displays all the plots of the various configurations.
The Title and Y-Axis fields support build variables.
Also a few changes in the plot area to have builds display names on the X-Axis instead of the build number (this is hardcoded).

The main code change is about separating the plot configuration and the plot data (or instance). Everything was managed in the Plot.java file. This is now split into Plot.java for the configuration part and PlotData.java for the 'instance' part. This last class is the one writing and reading data into the CSV file.

Code could be better but the functionality works. It has been in production in my shop for a few weeks and nothing to report.
Further changes to support more configuration options and variables into fields should be straightforward from here.

Cheers.