Python3 support

[superbobry]

I think we should follow Django guidelines on Python3, since the next release will support it.

[jzelinskie]

I asked about this on IRC previously, too. I really need to read through all of those guidelines. It looks like models should look like this:

from __future__ import unicode_literals
from django.utils.encoding import python_2_unicode_compatible

@python_2_unicode_compatible
class MyClass(object):
    def __str__(self):
        return "Instance of my class"

[superbobry]

Yup, exactly.

[superbobry]

A note about Py3 is now in our README.md.