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``
Fixes issue #2769

The translate function in the latex printing module was not being called on
single variables in vector expressions. This is now implemented in the
mechanics latex printer so that variables show up correctly in Latex printing.
Note that gamma is reserved for gamma functions in SymPy and is not considered
a valid variable name for simple translation to its latex form.
Spelling and expansion of note.
Added one more test.
Added test to exhibit Matrices inablity to print greek letters.

This seems to happen with Greek letters are used as function names are elements
of Matrices.
Fix whitespace issues in mpmath
Use mpmath 0.18 final instead of 0.18-a1
Update mpmath docs
Fix mpmath index files
Merge branch 'master' into issue-2769
Applied super_sub to functions.
Added blank line.
First implementation of Airy functions
Renamed Airy functions
Latex printing of Airy functions
Mirror symmetries
Real-imag splitting of Airy functions