title: In Angular 2.0.0-beta.0 map() and filter() are missing from a form input's Observable
body: <p>In Angular 2.0.0-alpha.47 the <code>Observable</code> returned from calling  <code>formInput.valueChanges()</code> had all the higher order functions i.e  I could do something like this;</p>
  
  <pre><code>this.search.valueChanges
   .debounceTime(150)
   .map(x=&gt;return x+1)
   .switchMap(text =&gt; this.youtube.search(text));
  </code></pre>
  
  <p>I'm now using 2.0.0-beta.0 and the <code>Observable</code> returned from <code>this.search.valueChanges()</code> no longer has all the methods. I can only subscribe. I can no longer call map(), filter() etc.</p>
  
  <p>Does anyone know if this is the expected behaviour for form inputs now? </p>