Add schemas for some astropy.modeling models

As I am reading through this and trying out pyasdf I tried creating schemas for `astropy.modeling` models commonly used for WCS transformations. So here they are.

I think only models which don't define an analytical inverse need the `inverse` definition in the schema so only the polynomials have it here. If this is acceptable I'll add the rest of the polynomials.
Should these be examples like

```
p1 = models.Polynomial1D(1, c0=1, c1=2)
tree = {"transform": p1}
f = AsdfFile(tree)
f.write_to('poly_example.asdf')
```

or simply the contents of a poly_example.asdf, like in the above comment?