docs(Http): add docs for Http lib

Fixes #2442

modules/angular2/angular2.ts

@@ -4,6 +4,7 @@ export * from './annotations';
 export * from './directives';
 export * from './forms';
 export * from './di';
+export * from './http';
 export {Observable, EventEmitter} from 'angular2/src/facade/async';
 export * from 'angular2/src/render/api';
 export {DomRenderer, DOCUMENT_TOKEN} from 'angular2/src/render/dom/dom_renderer';