Added support for injecting the injector

If you for some reason need the injector to manually get instances (for example for conditional instantiation), you can now inject it just like any other object.

import {Inject, Injector, Singleton} from "diskajs";

@Inject(Injector)
@Singleton()
export default class ClassWithInjector {

  injector:Injector;

  constructor(injector:Injector) {
    this.injector = injector;  
  }

}