Introducing HtmlResource

Much cleaner implementation than the Partial I had earlier. There's
still room for adding more value by further reducing the code needed to
compile partials, but for now, I am happier with what I have.

https://github.com/kyoken74/gwt-angular/issues/57
Cleaning up the Module interface and AbstractModule
Now using nested bundles.
Found more code still using DataResource. Migrated to HtmlResource.