Use __future__ to enable floating-point division of integers in Python 2

The current version of this patch includes specifically:
1. enabling consistent Python 3 division behavior in all files where I found ` / `,
2. replacing `/` with `//` where it seemed like integer division is desired,
3. removing `int()` and `float()` conversions that were no longer necessary after division behavior changes.

I'd dare say this is minor yet we want that for the next release. @Arfrever suggested that it fixes the issue of rounding download sizes down in Python 2.