MAINT/STY: Standardize __future__ imports

The code base had a heterogeneous mixture of __future__ imports where
some modules would import division some would import print_function and
some would import absolute import, with this set of changes all the
imports look like this:

from __future__ import absolute_import, division, print_function

Fixes #351.
BUG: Fix print statements in doctests

Module level imports affect the execution of the doctests, hence
everything failed in the build of the previous commit of this branch.

Move everything from `print x` to `print(x)`.
Merge pull request #414 from ElDeveloper/issue-351

MAINT/STY: Standardize __future__ imports