Add doctest to with_metaclass()
Merge pull request #2391 from asmeurer/with_metaclass_doc

Add doctest to with_metaclass()
Mention ask_update.py in docstring of compute_known_facts()
Move logic out of sympy/assumptions/handlers/__init__.py
Use CommonHandler's AlwaysTrue/AlwaysFalse
Add sympy/assumptions/handlers/__init__.py
Add nonpositive/nonnegative predicates/handlers
Add fuzzy_or
Add zero predicate to the new assumptions system

Even though zero = ~nonzero & real exactly, this ended up being more work than
it should have been.
Don't assume that oo is real in the sets.

In the new assumptions, it won't be. Because of the singleton Reals, which is
created at import time, this is needed to make SymPy import at all in #2210.
Misc formatting in ask.py
sympy/assumptions: return None -> return
Misc formatting issues in sympy/assumptions/handlers