Make it possible to install typing for Python 2

[JukkaL]

We should have a way of installing typing on Python 2. There are at least two alternatives:

1. Add something like setup_typing_py2.py that installs typing (only on Python 2). Not sure what the script should be called, but we should probably have a fairly descriptive name to make clear that this doesn't actually install mypy. Maybe this should be available via pip install mypy-typing or something. Alternatively, make this work on Python 3 as well.
2. Use a separate repo for typing and install it using pip install typing or so (at least for Python 2, but maybe also for pre-3.5 Python 3). This makes it more difficult to change typing, so we may want to postpone this until the implementation is stable and reasonable complete. We need to figure out who actually owns typing on pip now (I remember that there was a discussion about this a while back but I was then in the middle of moving my family to a different continent and was distracted.)

The long term goal is 2, but we may want to start with 1 until we have a better understanding of how things should work.

See also the original issue from where this came from: #917

[gvanrossum]

We're almost there with the typing repo. I've sent a PR that updates that repo to the current state from mypy: https://github.com/JukkaL/typing/pull/47

[vlasovskikh]

@gvanrossum I ported ambv/typehinting to Python 2.7 a month ago (python/typing#160) and uploaded typing 3.5.0.1 to PyPI.

[gvanrossum]

Ah, much better. I'll use that. Jukka: maybe you should just delete your typing repo then?

[JukkaL]

I updated my typing repo to have a prominent pointer to @vlasovskikh's version at the top of the readme.