added copyright notice to CMakeLists
added gcov compile flags

The ENABLE_GCOV option (cmake -DENABLE_GCOV=ON ...) adds the required flags
to gcc/llvm to generate gcov instrumentation output.
extended travis.yml to do gcov

Added steps to .travis.yml to run a coverage analysis and upload the results
to coveralls.io.
fixed path typo in travis.yml
added coverage badge