PY3K: add ``from __future__`` imports to all Python files.
PY3K: apply 2to3 print fixer.
PY3K: apply all 2to3 default fixers except `future` and `print`.
PY3K: fix imports.
PY3K: fix print call in templating.py that made 2to3 crash.
PY3K: clean up demo/benchmarks.py
PY3K: fix string/bytes issue in templating utility.
Merge branch 'python3-support'

This fixes all the easy issues; building with Python 3.x now works (without 2to3).
There's some 2.x specific C code that needs fixing before the package can be
imported and used with 3.x.