Make satisfiable() work with arbitrary expressions, not just Symbols

This is in the same spirit as https://github.com/sympy/sympy/pull/2451.

This also should make satisfiable() more deterministic by sorting the symbols.
Fix spelling error in function name
Merge branch 'master' into satisfiable_expr

Conflicts:
	sympy/logic/tests/test_inference.py
Merge branch 'master' into satisfiable_expr
Merge pull request #2506 from asmeurer/satisfiable_expr

Make satisfiable() work with arbitrary expressions, not just Symbols