Started to refactor JS Plugin class

Start with making plugin.js into a module, and re-implementing Plugin
using the class-based ES6 syntax, as it's a lot cleaner. Dispatch
events from setters instead of using Object.observe when changing
values that prompt external changes.

Vulcanize's whitespace stripper doesn't like the ES6 syntax though, so
disable that.

Also start implementing some tests using Mocha and should.js.

Finish refactoring JS Plugin code

scripts/vulcanize.js

@@ -34,7 +34,6 @@ for (var i = 0; i < release_paths.length; ++i) {
 
     child_process.execFileSync(vulcanize, [
         '--inline',
-        '--strip',
         '--config',
         path.join(root_path, 'scripts', 'vulcanize.config.json'),
         '-o',