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/http.ts

@@ -0,0 +1,11 @@
+import {bind, Binding} from 'angular2/di';
+import {Http, HttpFactory} from './src/http/http';
+import {XHRBackend} from 'angular2/src/http/backends/xhr_backend';
+import {BrowserXHR} from 'angular2/src/http/backends/browser_xhr';
+
+export {Http};
+export var httpInjectables: List<any> = [
+  XHRBackend,
+  bind(BrowserXHR).toValue(BrowserXHR),
+  bind(Http).toFactory(HttpFactory, [XHRBackend])
+];