Add code coverage targets to the CMake file

In order to better determine the quality of the test suite, it would be good to have code coverage statistics. I have found this CMake module (https://github.com/bilke/cmake-modules/blob/master/CodeCoverage.cmake) which provides the functionality, but I have not come up with a good strategy for creating the targets needed to implement it. That's why I am adding the JIRA issue here.