angular/api: update ui asynchronously

/cc @stephenplusplus @s3ththompson

I think the input[text] is not showing as disabled because of color: inherit in common.css: https://github.com/tastejs/todomvc-common/blob/master/base.css#L123

Demo: http://async.gcloud-todos.appspot.com/examples/angularjs
Might as well inline the function here since we're not going to be re-using it.