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
This should not be changed here. Go in astropy_helpers and do:

```
git checkout a93515f3fad840e363d84ca1cc094aba7420f80b
```

then add astropy_helpers