Make mixin parameterized

This builds upon https://github.com/usepropeller/react.backbone/pull/13, so for the new commits, the commits of March 12 can be ignored.

In this PR I made the mixin parameterizable (fixes #11 ) , so we can listen to multiple models. createBackboneClass is compatible, but the mixin itself isn't. We could do this, but I think it's unadvisable to keep it (that is, if we do that, I would advise printing an warning).

Please tell me what you think!