feat(http): add basic http service

This implementation only works in JavaScript, while the Observable transpilation
story gets worked out. Right now, the service just makes a simple request,
and returns an Observable of Response.

Additional functionality will be captured in separate issues.

Fixes #2028

modules/angular2/src/facade/async.ts

@@ -107,5 +107,5 @@ export class EventEmitter extends Observable {
 
   throw(error) { this._subject.onError(error); }
 
-  return (value) { this._subject.onCompleted(); }
+  return (value?) { this._subject.onCompleted(); }
 }

modules/angular2/src/facade/collection.ts

@@ -238,6 +238,7 @@ export class ListWrapper {
       l.sort();
     }
   }
+  static toString<T>(l: List<T>): string { return l.toString(); }
 }
 
 export function isListLikeIterable(obj): boolean {