Added libomp formula for 64 bit OS X.

libomp is the OpenMP Runtime Library by Intel. libomp dylib and headers are needed in Xcode projects to write OpenMP programs using clang.
There is no binary included in this package. I'm currently working on a formula for http://clang-omp.github.io/, which will contain a clang compiler capable of compiling OpenMP code, where testing will be possible. But the only test for the library would be checking the existence of the files, which is rather pointless, isn't it?