MVP tests for hass-util.html (#629)

* MVP tests for hass-util.html

* MVP util.js to allow individual JS function export

* Use mocha to unit test js logic

* Isolate mocha test directory

* Move mocha opts to separate file

* Default export of util function

* Use reify for mocha tests instead of babel

js/util.js

@@ -0,0 +1,13 @@
+/**
+ * Export hass util functions to window.
+ *
+ * This file is a workaround for the fact that Polymer 2 doesn't work well with
+ * ES6 JS imports. Once we move to Polymer 3, we should be able to simply
+ * import these functions where we need them.
+ */
+
+import attributeClassNames from './common/util/attribute_class_names';
+
+window.hassUtil = window.hassUtil || {};
+
+window.hassUtil.attributeClassNames = attributeClassNames;