docs(Observable): add documentation for Observable and operators

Closes #5642
Closes #5684

modules/angular2/examples/facade/ts/async/observable.ts

@@ -0,0 +1,8 @@
+// #docregion Observable
+import {Observable} from 'angular2/core';
+var obs = new Observable<number>(obs => {
+  var i = 0;
+  setInterval(_ => { obs.next(++i); }, 1000);
+});
+obs.subscribe(i => console.log(`${i} seconds elapsed`));
+// #enddocregion