Scrutinizer code coverage and custom Makefile support

Relates to #189 and adds:

- [x] [Scrutinizer][1] code coverage, provided by [Respect/Foundation](http://github.com/Respect/Foundation).
- [x] Add [Scrutinizer][1] badges to README (code quality and code coverage).
- [x] Fix bug that caused PHP to start output (:shit: any further HTTP header definition).

[1]: https://scrutinizer-ci.com