Add aggregate-report that reports cross module code coverage for the maven plugin

We have had issues with gaining accurate cross module code coverage numbers with jacoco. For instance when code in module A is touched during tests from module B, in our case when they are both part of a multi-module build.

This pull request makes it possible to report coverage numbers on classes outside of the current module. By adding a list of sourceFolders and classFolders which is a list of source and class folders outside of the current module to be scanned for classes to report coverage on.

The aggregate-report then concatenates the jacoco.exec files from reactor projects so that a unified report can be created.