Python 3 Compatibility

removed the unused and unnecessary \__name__ == "__main__" code.
changed except statement to use "as" instead of a comma (works in py2 & 3).
added workaround for python3's missing unicode() funtion. you shouldn't use that to convert non-strings to unicode ;-)

i recommend a version bump (not only in setup.py, but in arff.py too), and the mention of dumps and loads in the usage.