Add support for Python 3

- Rebase serialization exceptions on TypeError
- Add a "compat" keyword argument as a shortcut for protocol=0,
  trans_except=False in the Python constructor.
- Use io.BytesIO instead of cString.StringIO and wrap the stream
  appropriately when using Python 3.
Switch PHP to a 2-stage loader

- Do not expect a prompt, just send a string to be evaluated: removes the
  readline requirement and fixes usage without a TTY in one go.
- Remove cruft from bond/PHP/__init__
- No need to escape comments in the stage2/prelude anymore, only in stage1.
- Move shared code into bond for other languages
2-stage loader for Python.