CMake, RBJ filters, unit tests and doc

Now unit tests are included which check that after an impulse input the filters relax to zero and won't generate NaN.
The RBJ filters now also have a "filter" command included which previously had to be added manually. With that addition the RBJs behave as all the other filter types and are no longer different in any way. Generally, a "setup" command does the trick. RBJ filters are widely used for audio processing. They are all 2nd order. Check them out.
The build system is now CMake so that for all different platforms just one build system is needed.
A couple of comments have been added to the header files to make it easier for doxygen to create meaningful documentation.
Both static and dynamic libraries are generated. For Linux/Mac the dynamic ones are recommended (.so) and for Windows the static one (.lib).