feat(ng-pluralize): Implement the ng-pluralize directive

This is an attempt to implement #133 - It is still **a WIP**

There's still one thing I can't figure out how to do: I [need to replace](https://github.com/angular/angular.dart/pull/355/files#diff-3661a08861b43c8621c21aa4dd8be173R47) `{}` with someting like `{{ count - 2 }}` but `count` is interpreted as a string and then `-` triggers an error: `NoSuchMethodError : method not found: '-'`

Would someone have a hint on how to solve this ?

Todo
- [x] Finish the implementation
- [x] Make sure it works with both num & string (whens & count)
- [x] Add tests
- [x] Remove the modified helloworld from the PR