Add support for injecting services in other services.

[aerotoad]

Issue: The dependency injection system is designed to work only from services to components. The @Inject() decorator might already work out of the box to inject a service into another, but the @Injectable() metadata object doesn't support a services array to keep them in the bundle.

Solutions:

• Investigate if the @Inject() decorator already allows to inject services into other services.
• Add a services array to the InjectableMetadata object like this:

@Injectable({
  selector: 'myService', 
  services: [MyOtherService]
})
export class MyService {

  @Inject() myOtherService: MyOtherService

}

[aerotoad]

It seems like it's possible to use @Inject() inside a service without having to change anything.
Apparently adding the service type as the property type is enough to keep the service from being removed in the build without the need to explicitly set it in a services array.

This needs further checking, because if that's the case, the services array could be removed from the @Component() decorator as well.