Run code coverage on your local machine

This change introduces a new make target called "coverage" it will
compile your code using special GCC flags, run the tests, and then
generate a nice HTML output.

This new make target will only be available if you build using GCC in
Debug mode.

To build the code coverage and open it in your browser do this:

    mkdir debug
    cd debug
    cmake -DCMAKE_BUILD_TYPE=Debug -DENABLE_COVERAGE=ON ..
    make -j20
    make coverage
    xdg-open coverage/index.html