More WCS convenience functions: pixel_scale, celestial

Some more WCS convenience attributes:

`wcs.pixel_scale` returns the pixel scale ("plate scale") in degrees in the celestial dimensions *if* the pixels are symmetric (otherwise, it raises an exception)
`wcs.celestial` returns a copy of the celestial component of the WCS

Question: Should `wcs.pixel_scale` return a `Quantity`?

cc @mdboom
From experience with APLpy, I think the correct approach is:

https://github.com/aplpy/aplpy/blob/master/aplpy/wcs_util.py#L83

This was the result of many iterations :)