Enable the no-unused-vars ESLint rule

Please see http://eslint.org/docs/rules/no-unused-vars; note that this patch purposely uses the same rule options as in `mozilla-central`, such that it fixes part of issue 7957.

It wasn't, in my opinion, entirely straightforward to enable this rule compared to the already existing rules. In many cases a `var descriptiveName = ...` format was used (more or less) to document the code, and I choose to place the old variable name in a trailing comment to not lose that information.

I welcome feedback on these changes, since it wasn't always entirely easy to know what changes made the most sense in every situation.

test/unit/testreporter.js

@@ -74,3 +74,5 @@ var TestReporter = function(browser, appPath) {
     setTimeout(sendQuitRequest, 500);
   };
 };
+
+exports.TestReporter = TestReporter;