enable openmp builds

[ericmjonas]

[timmeinhardt]

I extended the README.rst, setup script and general interface to support OpenMP multithreading. The num_threads parameter can be used to specify the number of working threads.

Adding support for OSX was a little bit tricky. The default OSX compiler clang (which is a special OSX version of clang and overwrites the gcc compiler) does not support OpenMP builds. Therefore, one has to brew-install an independent gcc compiler. For this case, the setup script looks for other manually installed gcc compilers and applies the most recent version for the compilation.

@ericmjonas The respective commit https://github.com/ericmjonas/pybm3d/commit/29c800a755700053da8f30669fae8d5d2dd107de includes a few hacky changes. Maybe you can have a look at the most recent version and test it.