FreeBSD initial support

[rayrapetyan]

What was wrong?

FreeBSD OS wasn't supported

How was it fixed?

Added necessary checks and provided dev installation instructions in readme file.

[carver]

Thanks! If you're interested, a circle test running in a FreeBSD container would help make sure that support doesn't break silently in the future.

[rayrapetyan]

@carver, thanks! Do you mean tox -e smth? Any idea how to pass

--global-option=build_ext --global-option='-I/usr/local/include'

into tox properly?

[voith]

@carver Doesn't seem like circleCI supports free bsd: https://hub.docker.com/r/circleci/python/tags/

Do you mean tox -e smth? Any idea how to pass

@rayrapetyan I think @carver wants to have the test suite run on a FreeBSD container in circleCI:
for eg: we have some code running on jessie:

web3.py/.circleci/config.yml

Line 179 in f1e8e91

- image: circleci/python:3.5-jessie

[carver]

Doesn't seem like circleCI supports free bsd

Thanks for checking that @voith ! That's exactly what I had in mind. Nothing more to add to this PR, then.