Add Python 3 support and fix a couple of small bugs

There were a couple of small issues that prevented `django-mysqlpool` from working on Python 3, so I fixed those. After that, I fixed a small bug in the `HashableDict` implementation, bumped the version number, added universal wheel support, did some documentation cleanup, tweaked the `.gitignore`, and made some style tweaks to be in line with pep-8 and pep-257.