Modify Python aggregator to support full aggregator names.

- Fix bug in Python error traceback.
- Rename interpreter::var() => interpreter::get()
- If interpreter::get() receives a dotted name, it tries to import
  the attribute instead of fetching it from current stack.
- examples/python.cpp modified.
- Bindings maps 'site-packages/audiopia_ex/' directory to 'audiopia.common' module.