Add linting for SASS and Javascript

Similar to how PEP8 enforces Python style, there are tools for front end code:
- jshint[1] for Javascript
- sass-lint[2] for SASS

This commit adds the Gulp plugins for both, and sets up some sensible rules
(which can be iterated on).

It also adds a command to `./scripts/run_tests.sh`, so that any errors in the
front end code will fail the build before it has a chance to be deployed.

1. http://jshint.com/
2. https://www.npmjs.com/package/sass-lint

scripts/run_tests.sh

@@ -25,9 +25,12 @@ function display_result {
 pep8 .
 display_result $? 1 "Code style check"
 
+npm test
+display_result $? 2 "Front end code style check"
+
 ## Code coverage
 #py.test --cov=app tests/
-#display_result $? 2 "Code coverage"
+#display_result $? 3 "Code coverage"
 
 py.test -v
-display_result $? 3 "Unit tests"
+display_result $? 4 "Unit tests"