Faster ``minpoly`` using composed sum.

Added power series functions on "ring.PolyElement" objects.
Update mpmath to 0.18-a1

The paths haven't been adapted yet.
Fix mpmath imports
small changes in the documentation; used ``as_int``
initial attempt
working on
Fix whitespace issues in mpmath
Use mpmath 0.18 final instead of 0.18-a1
Update mpmath docs
Fix mpmath index files
tests pass, refactory in progress
fixed substitution bug and added tests
hash function restored
merged with master
fixes
metric bug fixed, various refactories
inverse metric bug fixed; Kronecker Delta now can have data
tested TensMul.__call__ on contracted tensors
First implementation of Airy functions
Renamed Airy functions