Make installation under Windows smoother.

There are some issues when installing on Windows, i.e. a regular `pip install openhtf` doesn't work. In particular:
* Some of our dependencies have their own dependencies that can't be fully handled by pip.
* The `cryptography` package is pip installable, but needs extra pip flags and crashes pip install for openhtf.
* If there ultimately _are_ non-pip install pre-reqs, we should document them in README.md.

@agdeepak, can you add detail based on your recent attempts to install on Windows?