Add checks to maintain a consistent Python code style and prevent bugs (#515)

A flake8 configuration is included in setup.cfg and the checks are
executed automatically on Travis:

* Ensures a consistent PEP8 code style
* Does basic linting to prevent possible bugs
diff --git a/.travis.yml b/.travis.yml
index a56120b..ff4662f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -38,11 +38,12 @@
   # Documentation build:
   - os: linux
     language: docs
-    env: DOCS STYLE
-    install: pip install sphinx sphinx_rtd_theme
+    env: DOCS STYLE LINT
+    install: pip install sphinx sphinx_rtd_theme flake8 pep8-naming
     script:
     - make -C docs html SPHINX_OPTIONS=-W
     - tools/check-style.sh
+    - flake8
 cache:
   directories:
   - $HOME/.cache/pip