use setuptools extras to declare extra dependencies in the backend

[kalikaneko]

I think a cleaner solution for installing different backends would be to declare them in the form of setuptools extras.
if you're ok with that idea, I can submit a PR
https://stackoverflow.com/questions/4796936/does-pip-handle-extras-requires-from-setuptools-distribute-based-sources

[kalikaneko]

PR in #3