Add flake8 to Travis-CI configuration

.travis.yml

@@ -8,6 +8,8 @@ before_install:
 install:
   - pip install -r requirements.txt
   - pip install coveralls
+before_script:
+  - flake8 .
 script:
   - coverage run --rcfile=.coveragerc --source='.' -m py.test -vv --runslow
   - coverage report -m

CONTRIBUTING.rst

@@ -255,8 +255,10 @@ If you submit a bug report, we appreciate if you include the standard output of
 Code style
 ~~~~~~~~~~
 
-All code should try to adhere to the `PEP 8 style guide <https://www.python.org/dev/peps/pep-0008/>`_.
-An integrated ``PEP 8`` linter in your editor is recommended!
+We use the python source code checker `flake8 <http://flake8.pycqa.org/en/latest/>`_ to help us maintain a consistent style across the code base. It runs automatically as part of our `Travis-CI <https://travis-ci.org/JakobGM/astrality>`_ configuration.
+
+You can lint your code locally by running ``flake8 .`` from the root of the repository.
+Integrating ``flake8`` into your workflow is recommended, there are plugins available for most popular `IDEs <https://foxmask.trigger-happy.eu/post/2016/02/17/pycharm-running-flake8/>`_ and `text-editors <https://github.com/nvie/vim-flake8>`_!
 
 In addition to this, some additional styling conventions are applied to the project: