WIP: Add HorizonSystem tests

I would like to add tests for conversion to the HorizonSystem.

@dsberry Are there any existing reference results?

If not we could simply use a few observatory locations and a grid of dates?
Is this altitude/azimuth or azimuth/altitude? It seems like the files for pyephem and pyast are not consistent right now since the first column of this file goes from 0 to 360 but it's the second column that does for pyast?