Add initial support for Python 3

I don't consider this issue closed until Simon officially supports
Python 3. Once that happens, the tox.ini and .travis.yml files needs to
be updated so that it no longer pulls from Simon's `python3` tag. I'll
probably also bump the minimum version of Simon.

I'm also breaking up the tox.ini file so that it performs more
meaningful Flask version-based tests. Testing each specific release is a
bit much. Testing the most recent version of each minor version makes
more sense. Also, it is consistent with how Simon handles PyMongo
versions.

References #8
Remove reference to Simon's python3 branch

0cd16a1 added references to Simon's `python3` branch in order to test
against an unreleased version. Now that a version of Simon has been
released with Python 3 support, the tests can be run against the version
available on PyPI.
Add Python 3 to supported languages

Closes #8
Require a version of Simon that supports Python 3

Now that both Simon and Flask-Simon support Python 3, it's a good idea
to make sure that someone using both libraries is using versions of each
that support Python 3.
Update changelog
Add wheel support
Remove version requirements for testing libraries