Faster ``minpoly`` using composed sum.

Added power series functions on "ring.PolyElement" objects.
More efficient dispersion computation not using resultants
Docstrings for dispersion functions
Clean up generator handling
Define maximum of empty set
Add tests
Avoid factoring the same polynomial twice
Fix mathematical description
More and better tests for dispersion computation
An even better implementation of the dispersion algorithm
Add complete references
Fix a bug and factor over the correct domain
Construct test polys directly
Update mpmath to 0.18-a1

The paths haven't been adapted yet.
Move dispersion code to polynomials module
Proper documentation for all dispersion stuff
Fix mpmath imports
small changes in the documentation; used ``as_int``
add Set.boundary property
naive solution for Union._boundary