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.
allow overwriting existing files in export (thanks to Chris Burel)
Merge pull request #28 from MichaelBlume/future-division

use futuristic division
do integrity check when closing collection
use future division in those aqt files too
future imports must go at top
fix race in preview mode, and fix preview shortcut keys in qt5
handle chrome's newlines in url issue and make sure to clean up progress
update obsolote activated() signals
fix regression caused by preview code

we shouldn't blank out self.card when multiple cards selected, as a number of
other routines take it to mean the most recently selected card.
more robust desktop location detection
Merge remote-tracking branch 'blume/future-division' into flake

Conflicts:
	anki/sched.py
	anki/utils.py
	aqt/reviewer.py
Re-clean a bit.
Merge remote-tracking branch 'dae' into flake

Conflicts:
	aqt/browser.py
	aqt/main.py
	aqt/reviewer.py
	aqt/sync.py
Cleaned recently changed files.