WIP: LogPy

Introduces a LogPy based `refine` function.

This work replaces sympy's unify module with an external dependency, [LogPy](http://github.com/logpy/logpy)

This should not be merged.  It depends on the development/git version of LogPy.

You can see where this is going by looking at the comparison with the associated matrix expressions branch
https://github.com/mrocklin/sympy/compare/logpy-start...matrix-cookbook-logpy