Python 3.3 compatibility

Python code adjusted to run on Python 2.6+ and Python 3.3+ from the single codebase.
Changes are splitted on per-module basis to simplify future hunting on regression bugs, using `git bisect`.

Known major issue:
- Radau and Dopri integrators segfault in 64-bit Python 3 environments ([issue 26](https://github.com/robclewley/pydstool/issues/26))

Branch has been rebased on current 'master'.