bumping version to v0.3.4.dev for development
Updating docstrings and removing `get_POAV_attr` method from
_poav_atom module classes.
Replacing six.u function with unicode_literals import.
Adding six module to install_requires list.

Also updating PyPI classifiers to include Python 3.3.
adding tox.ini for tox testing
Handle exceptions due to missing GUI dependencies.
Handle ImportError in nanogenui script due to missing deps.
updating setup.py for release of version 0.3.4