implement flake8 setup

[tomkralidis]

Overview

This PR implements a basic flake8 check in travis, with some overrides (see setup.cfg), as well as fixes flake8 errors across the codebase.

Related Issue / Discussion

Additional Information

• line length is 120 characters
• top of file import warnings are ignored
• code in tests/ is excluded

Contribution Agreement

(as per https://github.com/geopython/pywps/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to PyWPS. I confirm that my contributions to PyWPS will be compatible with the PyWPS license guidelines at the time of contribution.
• I have already previously agreed to the PyWPS Contributions and Licensing Guidelines

[coveralls]

Coverage decreased (-1.0%) to 75.573% when pulling 6c498c2 on tomkralidis:flake8 into 45326f7 on geopython:master.

[coveralls]

Coverage decreased (-0.1%) to 76.432% when pulling 2fdbb69 on tomkralidis:flake8 into 45326f7 on geopython:master.

[coveralls]

Coverage decreased (-0.1%) to 76.432% when pulling 2fdbb69 on tomkralidis:flake8 into 45326f7 on geopython:master.