Simplifies tox.ini & adds MySQL + Python 3.
Removes a useless arg in tox.
Uses tox in Travis CI.
Fixes MySQL + PyPy3 build.
Excludes MySQL + PyPy3.
Subdivides Travis CI into jobs to lighten builds.
Merge pull request #15 from BertrandBordage/master

Simplify tox.ini, add MySQL + Python 3, use tox on Travis.