Remove unnecessary with_statement imports.

Iris requires Python 2.7, so the with statement is already enabled.
Add __future__ imports on all remaining files.

All these files should not be affected by these imports, so this is just
one big mass change. The files where these imports make a difference
were already changed in a previous commit.

Only the examples are exempt from the __future__ imports; this should make
things simpler for users, plus they have no real effect in these files.
Add coding standards test for __future__ imports.
Merge pull request #1374 from QuLogic/python3

Add remaining Python3 __future__ imports