Add GCOV/Coveralls integration scripts.

Add in your CMakeLists.txt:

.. code:: cpp

  if(ENABLE_GCOVERALLS)
    include(GCoveralls)
    enable_gcoveralls()
  endif()

And in your .travis.yml:

.. code:: yaml

  - cd build
  - cmake .. -DENABLE_GCOVERALLS=1
  - make
  - make test
  - ./coveralls-upload