Fix for #33: null in attribute value breaks XML AttValue

In several places we were only checking for null in the state's
if-statement and not breaking out of charsUntil for it.
reuse README.rst on PyPI
base trove classifiers on releases tested by Travis
explicitly require six in setup.py
use charade instead of chardet for Python 2.6 - 3.3 compatibility
add tox.ini to simplify local tests on multiple Python versions