TST: Add flake8 testing for new additions

Add flake8 testing for new additions
Only applies to main code for now, not test code

.travis.yml

@@ -104,7 +104,7 @@ before_install:
 
 # Install packages
 install:
-  - conda create --yes --quiet -n statsmodels-test ${PKGS} ${OPTIONAL} nose pip pyyaml setuptools
+  - conda create --yes --quiet -n statsmodels-test ${PKGS} ${OPTIONAL} nose pip pyyaml setuptools flake8
   - source activate statsmodels-test
   - if [ ${COVERAGE} = true ]; then pip install coverage coveralls; fi
   - python setup.py install
@@ -118,7 +118,12 @@ script:
   # Compose a script to run testing with coverage
   - echo 'import statsmodels as sm; a=sm.test(label="full", verbose=3); import sys; sys.exit((len(a.failures)+len(a.errors))>0)' > test.py
   - if [ ${COVERAGE} = true ]; then coverage run --rcfile=${SRCDIR}/.travis_coveragerc test.py; else python test.py; fi
+  - cd ${SRCDIR}
+  - git remote add upstream https://github.com/statsmodels/statsmodels.git
+  - git fetch upstream
+  - find . -type f ! -name "test*.py" -name "*.py" -path "./statsmodels/*" | xargs git diff upstream/master | flake8 --diff
 
 after_success:
   # Coverage only for 2.7
+  - cd "${SRCDIR}/travis-test"
   - if [ ${COVERAGE} = true ]; then coveralls --rcfile=${SRCDIR}/.travis_coveragerc; fi