Preliminary typing.py, anticipating provisional acceptance of PEP 484.

Preliminary typing.py, anticipating provisional acceptance of PEP 484.

There area bunch of TODOs here, but the biggest (not mentioned in the
file) is that I'm going to take out __instancecheck__ and
__subclasscheck__.  However my personal schedule is such that I
probably won't have time for these before Larry tags beta 1.  But I
will try -- this commit is mostly to make sure that typing.py doesn't
completely miss the train.

PS. I'm tracking issues at https://github.com/ambv/typehinting/issues.