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.
Reading the previous case [multidimensional  1D polynomial] shouldn't we keep that out of the ASDF framework as a shortcut? I guess I would expect a list of polynomial models in that case. For really large cases (like nonlinearity where each pixel has a polynomial, it just doesn't use this approach. If it is really that large, we should find a different way to store it. Is there good counter example?