cmake: add option for GCC profiling

using `-ggdb3` and `-gp` with GCC enables verbose debugging and
profiling abilities useful for further analysis; also the tests will get
compiled with `-ftest-coverage` if both CMake variables
(`pfasst_BUILD_TESTS` and `pfasst_WITH_GCC_PROF`) are enabled and you
are compiling with GCC
cmake and tests: enable and provide test coverage analysis

Using lcov and genhtml and a little shell script we are now able to
generate test coverage results with a single command (assuming one has a
build available with GCC)
Merge pull request #78 from torbjoernk/feature/profiler-support

Sweet!