Clearsky airmass modules

It would be best to complete Sandia-Labs/PVLIB_Python#17 before attempting to pull this. As @bmu suggested in Sandia-Labs/PVLIB_Python#10, we should go further, but I'm advocating for baby steps at this point so that we can keep merges manageable. 

This adds a ``get_solarposition()`` function back into ``solarposition``. I did that so that I could easily put a solar position calculator option in ``clearsky.ineichen()`` and possibly other places as well. 

I added a notebook file to /tutorial/ that demonstrates the modules basic functionality. I don't care if it survives in the end.

I don't think that this should break any existing functionality.