Added the option to specify the order of the derivative to polyder.

It would be neat to specify the order of the derivative in `polyder`.  Let me know what you think of my implementation.

EDIT: I also updated the array versions of `polyder` and `polyint`.
I'll merge this, but I'll add a specialized method for 1d that uses the comprehension. Also, since you care about this use case, maybe you could add a test for non-1d arrays of polynomials.