Encode the payload with UTF-8 to ensure socket can send the data.

Add six for Python 3 compatibility.

Update tests and use tox to test Python 2.7, 3.3, and 3.4. Python
3.2 is not supported since unicode strings are not valid.
flake8 fixes
Require six in setup.py
six dependency has moved to setup.py
Support Python 2.6
Merge branch 'python3' of https://github.com/stanhu/dogstatsd-python

See #18.
Move the requirement for six to tox as it is not referenced outside of tests.