Add cross module code coverage for 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.

This also adds a aggregate-report which concatenates the jacoco.exec files from reactor projects so that a unified report can be created.