Add OpenMP support to forward modeling modules in gravmag.

Giving decent speedups in my linux laptop and has no problem with `multiprocessing`.

Seems there might be some issues building the extension modules on Windows. Anaconda doesn't include OpenMP in its MinGW. See [this discussion](https://groups.google.com/a/continuum.io/forum/#!topic/anaconda/c-4u6C29hYM) and the [instructions here](http://docs-windows.readthedocs.org/en/latest/devel.html#mingw-with-openmp-support) for a possible solution.

This is an addon for PR #105 

- [x] Docstrings
- [x] Make tests for new code
- [x] All tests pass
- [x] Can be merged
- [x] Changelog entry
- [x] Benchmark
  - [x] prism
  - [x] polyprism
  - [x] sphere
- [x] Include new dependencies in Windows install docs