Merge pull request #1804 from mrocklin/strat-refactor

Refactor strategies
Fix a test to test the right thing

See https://github.com/sympy/sympy/pull/1803#issuecomment-13903307.
Merge branch 'master' of github.com:sympy/sympy
Merge pull request #1811 from mrocklin/random-parameters

Bugfix in sympy.stats - add support for random parameters
add Naturals0
add drv and PoissonDistribution
solve, sample, docstring in discrete
add Geometric Distribution
density -> distribution in frv.SingleFinitePSpace
refactor symbol, value, distribution to SinglePS
move _value_check to rv
single discrete pspace
hack compute_density in discrete
discrete distributions act like Lambdas
docstrings for discrete distributions
import stats/discrte into __init__
evaluate=False in discrete
test_args for discrete
next(...) -> .next() in test_fancysets
remove excess whitespace