refactor(http): use Observables in Http backends

BREAKING CHANGE: Http now returns Rx Observables directly, so calling .toRx() is no longer necessary. Additionally, Http calls are now cold, so backend requests will not fire unless .subscribe() is called.

closes #4043 and closes #2974

Closes #4376

modules/examples/src/http/http_comp.ts

@@ -1,6 +1,5 @@
 import {Component, View, NgFor} from 'angular2/angular2';
-import {Http, Response} from 'angular2/http';
-import {ObservableWrapper} from 'angular2/src/core/facade/async';
+import {Http} from 'angular2/http';
 
 @Component({selector: 'http-app'})
 @View({
@@ -16,8 +15,5 @@ import {ObservableWrapper} from 'angular2/src/core/facade/async';
 })
 export class HttpCmp {
   people: Object;
-  constructor(http: Http) {
-    ObservableWrapper.subscribe<Response>(http.get('./people.json'),
-                                          res => this.people = res.json());
-  }
+  constructor(http: Http) { http.get('./people.json').subscribe(res => this.people = res.json()); }
 }

modules/examples/src/jsonp/jsonp_comp.ts

@@ -17,7 +17,6 @@ import {ObservableWrapper} from 'angular2/src/core/facade/async';
 export class JsonpCmp {
   people: Object;
   constructor(jsonp: Jsonp) {
-    ObservableWrapper.subscribe<Response>(jsonp.get('./people.json?callback=JSONP_CALLBACK'),
-                                          res => this.people = res.json());
+    jsonp.get('./people.json?callback=JSONP_CALLBACK').subscribe(res => this.people = res.json());
   }
 }