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.
I can build it with `gcc-4.9`, just skip it's own dependency test.
The build script will check libiomp5 linked specific library. But building with homebrew's gcc will linked it's own library additionally. The test script detect additional library will raise error.
See my [script](https://gist.github.com/mbcoguno/92afce50ca197820cd3a) for more details.