[JENKINS-20706] Support matrix builds

by copying and parsing coverage.xml files from each MatrixRun.

This is my second attempt at addressing
https://issues.jenkins-ci.org/browse/JENKINS-20706

This works by creating a MatrixAggregator that on endRun, copies the
coverage.xml from each MatrixRun into the root build's directory. Then
in endBuild, those files are parsed and aggregated.

Refs:
* https://issues.jenkins-ci.org/browse/JENKINS-20706
* https://github.com/jenkinsci/cobertura-plugin/pull/26 (this PR)
* https://github.com/jenkinsci/cobertura-plugin/pull/22 (older PR)