Make public and document GenericMeta

[ilevkivskyi]

Fixes #335

I added GenericMeta to __all__ and added docstrings to GenericMeta and GenericMeta.__new__ following the discussion in the issue.
I decided not to describe __origin__ and friends in docstrings. Instead, I just added a comment at the place where they are first used.

[gvanrossum]

Harmless though this is I think it's also not critical to get it into 3.6.0. People who want it can use from typing import GenericMeta, which works regardless.