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.
Yes, each model is applied to the same input dimensions. In the 1D case:
```
p1 = models.Polynomial1D(1, c0=[1,2], c1=[3,3])
x = np.array([[1,2,3,4]]).T

p1(x)
array([[  4.,   5.],
       [  7.,   8.],
       [ 10.,  11.],
       [ 13.,  14.]])
```
A similar result can be achieved with a combination of remap_axes and a join operation and two separate instances of the polynomial model but there're use cases when this syntax is very convenient.