add trivial unevaluated Density class
not-all-args-basic issue

preorder_traveral takes function to determine parent
xreplace stops on atoms
rec_replace stops on non Basics
Interval.free_symbols does not include True/False
add ContinuousDensity and NormalDensity

replace .density with .pdf in standard code

Conflicts:

	sympy/stats/crv_types.py
	sympy/stats/rv.py
stats: FooPSpace -> FooDensity

This is a large commit.

The information for the continuous distributions was pushed from a
pspace object into a density object.  Probability spaces are still the
base of the logic of stats, there are now just fewer of them.

Sampling had to move to the density level which brought along cdf and
inverse cdf.

Docstrings have not yet been updated
fix Frechet
_print_RandomSymbol = _print_Symbol
Clean Finite Stats, RandomSymbol(Expr)

FooFinitePSpace -> FooFiniteDensity
Clean up domains and pspaces so that type(obj)(*obj.args) == obj

RandomSymbol inherits from Expr, not Symbol
mark some stats tests as slow
fix conditional finite domains and

also rv_subs uses subs->xreplace
RandomSymbols hold string, not Symbol
subs works on empty substitution
compute_innermost does not dive within nonexprs
RandomSymbol again owns symbol.  No longer atom
remove duplicate (and errant) docstrings in frv
fix typo in QuadraticUDensity docstring
rename FooDensity -> FooDistribution
add cdf to ContinuousDistribution
expectation and cdf to continuous distribution
test_args for old classes
add new classes to test_args