Make floats in proj4 strings be a fixed precision.

Previously, the default precision was used, but Python 2 and 3 differ on
what that is. This causes small variations in the results.

This change is expected to fail since tests are not updated.
Update proj4 test strings with increased precision.
Update Robinson tests for increased precision.
Update Transverse Mercator tests for new precision.
Update Lambert Conformal tests for new precision.
Update linear ring test for new precision.
Update plots for new precision.
Merge pull request #497 from QuLogic/fixed-precision-proj-strings

Fixed precision proj strings.
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