Implement __contains__ for SingleFiniteDistribution.

Fixes issue 3964: Test failure in master: __contains__ deprecated in test_finite_rv.py
https://code.google.com/p/sympy/issues/detail?id=3964
Properly reset warning filters in tests.

Fixes issue 2492: Better way to test warnings
https://code.google.com/p/sympy/issues/detail?id=2492
Increment stacklevel when wrapping warnings.warn().
Print enhanced asserts' lhs and rhs each on its own line.
Add missing __future__ import print_function.
Merge branch 'master' into tests2
XFAIL test_args entry for TransformationSet (now deprecated).
Properly deprecate TransformationSet.
Merge pull request #2364 from jrioux/tests

Fix test failures in master