4 ocean vertical coordinates factories.

@bjlittle This PR supersedes #1166.  It extends the AuxCoord factories of Iris by supporting the following CF dimensionless ocean vertical coordinates:

- ocean_sigma_coordinate
- ocean_s_coordinate
- ocean_s_coordinate_g1
- ocean_s_coordinate_g2

They where implemented in a "hackish" way by copying [OceanSigmaZFactory](https://github.com/SciTools/iris/pull/509) logic, so there is room for improvement!

Also, there are no unit tests, but I will try to implement something if iris devs think this PR is worth pursuing.

@rsignell-usgs Here is a notebook showing all 4 coordinates in action that needs your expert opinion:

http://nbviewer.ipython.org/gist/ocefpaf/18e8fe29005c148fe0e3