Merge pull request #1749 from mrocklin/rewriterule-condition

Rewriterule condition
Merge pull request #1720 from mrocklin/stats-rewriterules

Refactor sympy.stats
add statsimp
factor out common Density call in rewriterules
density -> distribution in simplify
add unpack_Density rule
Use rebuild rule.  Simplify tests
Testing statsimp

chisquared example works
testing expression simplification separately from densities
changed strategies in statsimp
factor out additive versions of rules
statsimp uses top_down and factor