Prepare for py3k

Make a start toward #91 (we are still blocked on the `protobuf` dependency).

Selectively apply patches suggested by 'python-modernize --six-unicode'.

Tweak the pylint messages suppressed in test/demo code to work around issues involved in "straddling" Python2/Python3:
- The `six` module does some import hackery which confuses pylint when used in a virtualenv:  suppress `import-error` check (only in test/demo code).  Note that the check is redundant (we have 100% coverage) as well as failing bogusly.
- Rather than introduce `from __future__ import print_statement` in demos, just use `print()`, which is legal in Python2, and forward-compatible to Python3, but causes a `superfluous-parens' warning under pylint:  suppress that warning (only test/demo code).
A few things

1. If we support it, we should test it in Travis
1. We should be consistent, either always or never use the future import. (So would be never if we want to support 2.5.)
1. The `__future__` imports are not wonky and are in place for our exact issue (codebases that straddle Python versions)