Initial implementation of inject.dart support (BindingAnnotation)

This patch makes dado use inject's BindingAnnotation to recognize metadata that define bindings (now only BindingAnnotation metadata is used by dado as opposed to any metadata). Named class has been included as an easy to use BindingAnnotation.

Probably you guys have already done this internally, but in case you didn't here it is. 

Actually, I made this to ask you if I may contribute to this project. May I proceed with the implementation of @inject?