Add mypy type linting

Author: Drvanon

travis.sh

@@ -3,10 +3,12 @@
 set -e
 
 pip install pylint
+pip install mypy
 
 if ! [[ $NOPANDAS -eq 1 ]]; then
     pip install pandas
 fi
 
 nosetests --with-coverage --cover-package=functional
 pylint functional
+mypy functional