refactor(build): simplify and modularize

simplify:
- use same html file for dart and JS
- build benchmarks automatically when doing `gulp build`
- centralize configuration

modularize:
- move all build tasks into separate node.js modules under
  `tools/build`.

changes:
- the `build` folder is now the `dist` folder

Closes #284

karma-dart.conf.js

@@ -1,6 +1,6 @@
 // Karma configuration
 // Generated on Thu Sep 25 2014 11:52:02 GMT-0700 (PDT)
-var file2moduleName = require('./file2modulename');
+var file2moduleName = require('./tools/build/file2modulename');
 
 module.exports = function(config) {
   config.set({