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``
Fix whitespace issues in mpmath
Use mpmath 0.18 final instead of 0.18-a1
Update mpmath docs
Fix mpmath index files
First implementation of Airy functions
Renamed Airy functions
Latex printing of Airy functions
Mirror symmetries
Real-imag splitting of Airy functions
Complex expansion
Hypergeometric representation of Airy functions
Function expansion for special arguments
Taylor series expansion for Airy Ai and Bi
Numerical evaluation of airy primes
Rewrite Airy functions in terms of Bessel functions

Note: Maybe we should not implement these formulae but
rewrite to hyper first and then call hyperexpand? This
will however allow for less explicit control.
Fix mathematical errors in the expansion formulae for Airy(c*(d*z**n)**m)