Add PEP8 checker to TravisCI

.travis.yml

@@ -15,9 +15,11 @@ matrix:
       env: NOVA_DIR=nova3
 
 install:
-  - pip install pyflakes
+  - pip install pycodestyle pyflakes
 
 script:
-  - cd "$TRAVIS_BUILD_DIR"
-  - python -m compileall "$NOVA_DIR/engines/"*.py
-  - pyflakes "$NOVA_DIR/engines/"*.py
+  - cd "$TRAVIS_BUILD_DIR/$NOVA_DIR/engines"
+  - python -m compileall *.py
+  - pyflakes *.py
+  # skipping E265, fixing it will break plugin usage on older qbt instances (< v4.1.2)
+  - pycodestyle --ignore=E265,W503 --max-line-length=100 --statistics *.py