Add __future__ imports to remaining Python files.

It should be known that these cause no difference since Cartopy has been
tested on Python 3. However, the lack of consistency makes for easily
introduced bugs in the future.
Rename test_pep8.py -> test_coding_standards.py.

Also, add license. Dates determined from git log.
Add coding standards test for __future__ imports.
Merge pull request #496 from QuLogic/python3-__future__

Add remaining Python3 __future__ imports