integrate coverage report to coveralls

[cgcostume]

already started in refine_test_ci branch.

[cgcostume]

Coveralls shall display the test coverage of probably non generated code as well as some regression tests... Travis should activate coverage reporting only in a single build path within the build matrix, probably gcc in release mode. For it, the cmake flag OPTION_TEST_COVERAGE=On should be used (already integrated in the refine_test_ci branch).

[cgcostume]

Coveralls is also setup. Only the shield needs to be added when integration is done:

[![Coveralls](https://img.shields.io/coveralls/github/cginternals/glbinding.svg?style=flat)](https://coveralls.io/github/cginternals/glbinding)