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 was not aware of the performance overhead.  My intention was to make `polyder` and `polyint` work for arrays of arbitrary dimension, but it is not essential for my needs and I can revert this change.