Port b-secret to Polymer Dart

- removed CustomEvents
- added data binding for value


### Known issue:
"required" wouldn't work:  if "required" is set on an input element it is always required:
e.g.  `<input required="false">` is required, `<input required>` is required
`<input>` is not required
Good questions.

Yes, you can serve a polymer app via the command-line. There are two important commands:

`pub serve` runs a local web server and `pub build` which is used to generate a deployable version of your Polymer.dart app. This already allows us to develop Polymer.dart apps seamlessly without the DartEditor.

Run `pub server -h` and `pub build -h` for detailed usage information.


Regarding the paths in `build.dart` and `pubspec.yaml` only the entries on pubsepc.yaml are necessary to run the application. It's because for using Polymer.dart there are still a couple of things which need to be compiled/transformed which is done by transformers (feature by pub). In order to let pub know how to transform the app properly the Polymer entry points (usually index.html) have to be defined.

If you take a closer look at the `build.dart` you will see that we call the function `lint` there. So everytime a file changes Polymer.dart specific linter will analyse the application and print out relevant debug information into the console. Since `build.dart` is called everytime a file inside the DartEditor changes, this currently is not very helpful without the editor.