[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%28hudson.matrix.MatrixRun%29), 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%28%29), those files are parsed and aggregated.

The advantage of this approach is that it seems to aggregate the coverage in a nice way -- i.e.: if my MatrixBuild does a MatrixRun for Python versions 2.6, 2.7, and 3.3 and each of those has lines that are not covered but all of the lines are covered by at least one MatrixRun, then the coverage for the overall build will be reported as 100%.

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)
* https://groups.google.com/forum/#!topic/jenkinsci-dev/yoamyzbqVro

Cc: @ssogabe, @kinow, @emanuelez, @kohsuke 

test