Future import division

This adds `from __future__ import division` to most files. This gets us closer to Python 3 and doesn't cost much. "unicode_literals" will probably be added by `unicode` (#23); this branch is merged in `unicode`.

The behavior changed by this is:
* `/` (single forward slash) is **always** the floating-point division, e.g. `9/2` is `4.5` not `4`, no need to write `9/2.0`
* `//` (two forward slashes) is now **the only correct way** to do integer division, e.g. do `9//2` if `4` is the result you need.