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

In Python 2, the division ('/') operator defaults to integer
(truncating) division when given integer argument. In Python 3, it
performs floating-point division unconditionally instead. To overcome
this difference and get a consistent behavior, integers were converted
to floats explicitly in a few places.

Instead, use a simpler 'from __future__ import division' statement that
enables floating-point division globally in Python 2. Use it in all
relevant files to get a consistent behavior, and use '//' appropriately
whenever integer division is desired.

X-Pull-Request: https://github.com/gentoo/portage/pull/2