For some reason the doc build is complaining about not having pages for the Pix2Sky/Sky2Pix base classes.  Unfortunately the only obvious way to add them is to include them in the module's __all__ even though they're not generally useful for users (though I guess useful as base classes for new projection types?)  If there's another way to do this let me know.  This is okay in the meantime though.
ModelMeta now creates a nicer __init__ definition for models that have no parameters.
Add missing support for keyword arguments to Pix2Sky_AZP
Reorder some of the API docs a bit.