Freeze pip dependencies to requirements.txt

Should work in python2's `pip install` now.

Unfortunately, default ubuntu/mint install doesn't have the
necessary *-dev dependencies by default to install stuff like
cryptography and pip will spit out cryptic error messages
about missing C headers, but google is informative if you search for the
error message.
Document pip install in readme

Also document old autobahn version dep.
Requirements: Minimum versions, no transitive deps.

Allow older versions, as they are currently the libraries used in
testing/development. In adition, remove transitive dependencies.
README: Debian install instructions.
Merge branch 'pr/1'