[JENKINS-20706] Support matrix builds

by copying and parsing `coverage.xml` files from each [`MatrixRun`](http://javadoc.jenkins-ci.org/hudson/matrix/MatrixRun.html).

This is my second attempt at addressing
https://issues.jenkins-ci.org/browse/JENKINS-20706

This works by creating a [`MatrixAggregator`](http://javadoc.jenkins-ci.org/hudson/matrix/MatrixAggregator.html) that on [`endRun`](http://javadoc.jenkins-ci.org/hudson/matrix/MatrixAggregator.html#endRun(hudson.matrix.MatrixRun)), copies the `coverage.xml` from each [`MatrixRun`](http://javadoc.jenkins-ci.org/hudson/matrix/MatrixRun.html) into the root build's directory. Then in [`endBuild`](http://javadoc.jenkins-ci.org/hudson/matrix/MatrixAggregator.html#endBuild()), those files are parsed and aggregated.

Refs:
* https://issues.jenkins-ci.org/browse/JENKINS-20706