Piecewise._eval_interval for cond. indep. of integration var.

This fixes the Integral of a Piecewise when the conditions are
independent of the integration variable.

Before:

>>> integrate(Piecewise((0, Eq(y, 0)), (x*y/sqrt(y**2), True)), (x, 1, 3))
4*y/sqrt(y**2)

After:

>>> integrate(Piecewise((0, Eq(y, 0)), (x*y/sqrt(y**2), True)), (x, 1, 3))
Piecewise((0, y == 0), (4*y/sqrt(y**2), True))
Fix issue 3649

The problem was that Integral._eval_subs was returning an instance of
Integral, so that NonElementaryIntegral.subs() was returning an instance of
Integral, and hence failing an isinstance check. I changed all methods of
Integral to use self.func(*args) instead of Integral(*args).  But we need to
audit all of SymPy and make sure we are doing this everywhere.  See issue
3652.
Fix tests in Python 3.3 with --no-subprocess

See issue 3629.  Test by running ./bin/test --no-subprocess in Python 3.3.
Update wiki references (point to githib/wiki)
Merge branch 'master' into old-wiki-refs
polarify doesn't descend beneath exprs
support rvs in rvs
check pspace always receives symbol

This is a common fail-point for poorly behaved SymPy Expr code.
density evaluate=False
RandomSymbol._diff_wrt
solvers._ispow checks isinstance Expr
Density(constant) -> Lambda(DiracDelta)
RandomSymbol.is_Atom
given simple case of Eq(Rv, expr) -> subs
ProductContinuousPSpace subs in all symbols in pdf
conjugate prior test
add FiniteDensity(dict)
Merge branch 'master' into remove-old-wiki-refs
Drop references to
https://github.com/sympy/sympy/wiki/old-wiki-Sympy-Development
Drop obsoleted refs to
https://github.com/sympy/sympy/wiki/old-wiki-Discussion