Make zxcvbn py2 and py3 compatible in one codebase using "future" packag...

...e

Hey all. I noticed this library hasn't been touched in a while, nor updated to work with py3. We'd like to use it with our py3-based app, so I took a quick pass with the "future" package to allow this python-zxcvbn package to be usable in py3 and py2.

I didn't see much testing, but did some manual testing and everything seemed OK.

I've not actually done this sort of py2/py3 single-source library before, so feedback welcomed (along with, of course, merging back into the main project and pushing to PyPI). 

Note the new dependency on the future package, which may or may not be an issue.