use futuristic division

Python used to use C-style division, where division of two ints was
truncated, and division involving a float resulted in a float.

This is confusing, because you often can't tell from looking at a
line of code in isolation what sort of division it's supposed to do.

With ```from __future__ import division``` Python ensures that division is
always explicit.

```//``` means (floored) integer division
```/``` means float division

regardless of argument types.

This should make the source a bit clearer now, as well as removing one
obstacle if Anki is ever ported to Python 3.