change frame aliases to simpler _name attributes on the frames themselves
drop CLASS_TO_NAME_MAP in favor of _name attribute
fixes to simplify SkyCoord using 1-to-1 frame-to-name mapping
_name -> name for frames
Make TransformGraph._cached_classes into public frame_set

@taldcroft's suggestion
add test for transforming SkyCoord with None frame
Merge pull request #30 from eteq/coordinates-ape5-1to1-framenames

Make frame string names  1-to-1 map to frame classes