Use builtins.range from the future library

I saw `Programming Language :: Python :: 3.3` in setup.py classifiers and thought librosa is Python 3 ready... It turns out it isn't, but there's an open issue for it (#86). I assume the link there is the author's preferred porting guide, so I used the future library.

This change makes *the parts I need* run under Python 3. Hopefully it's useful. Unfortunately I don't have "matlab" so I can't run the tests.