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

package.json

@@ -6,7 +6,7 @@
     "node": "5.0.0"
   },
   "scripts": {
-    "test": "echo \"Error: no test specified\" && exit 1",
+    "test": "gulp lint",
     "postinstall": "./node_modules/bower/bin/bower install",
     "build": "gulp",
     "watch": "gulp watch"
@@ -35,5 +35,11 @@
     "gulp-uglify": "1.5.1",
     "jquery": "1.11.2",
     "query-command-supported": "1.0.0"
+  },
+  "devDependencies": {
+    "gulp-jshint": "2.0.0",
+    "gulp-sass-lint": "1.1.1",
+    "jshint": "2.9.1",
+    "jshint-stylish": "2.1.0"
   }
 }