Don't rely on deprecated relative imports, use absolute imports.

I noticed the code had relative imports in them, but that's not very safe as global modules might exist with the same name. I converted it to use absolute imports. You could also use 'from .foo import .." for relative imports, but I don't know whether you want to be compatible with older versions  of Python 2.x which don't support that.

I tried to run the tests and I get a failure, but I don't think it's connected to my changes.