Add Py3 patches.

* Currently uses the future module for py2k compatibility. Only a couple of
  functions (just ``range`` and ``str``, I think) are used, so this dependency
  could be removed easily enough.
Add RKbug.py fixes

* Why is the diff so noisy??
More fixes on Py3, including the examples
Mention Py3 compatibility in setup.py.

* Also add future==0.4.0 to requirements.txt. This is not currently read; it is
  for the user's info only ...