Fix errors with Python 3.3.2 on Ubuntu 13.10

external/prettytable.py
- Import StringIO from cStringIO (Python 2) or io (Python 3)

farray/__init__.py
- Use relative imports

flabel.py
- Use itertools.izip for Python 2 only
Merge pull request #72 from khaeru/master

First steps in removing 2to3