Added libiomp5 formula

libomp is the OpenMP Runtime Library by Intel. libomp dylib and headers are needed in Xcode projects to write OpenMP programs using clang.
Just to clarify things: The primary build result of this formula has the name `libiomp5.dylib`. The header files are named implementation independent (`omp.h` et al).
If you would like a different name for the formula, that's fine with me.