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.