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.
@nden: Yes, I think the format here should be as straightforward as possible.  So no `degree`, just an array of coefficients.  I think multidimensional polynomials would also be desirable -- in that case, maybe change the name of this tag?