title: No chance to make an strongly typed object observable by mobx
body: <p>I use mobx library. It fit well with ReactJS.
  I have and observable array like this:</p>
  
  <pre><code>@observable items = [];
  </code></pre>
  
  <p>When I add an object in this way, I have no problem and the given object will be observable as expected.</p>
  
  <pre><code>let x = {
  Title: "sample title"
  }
  
  items.push(x);
  </code></pre>
  
  <p>but when I define an strongly typed object (using typescript)</p>
  
  <pre><code>export class SampleObject {
      Title: string;
      constructor(title: string) {
          this.Title = title;
      }   
  }
  </code></pre>
  
  <p>and pushing new object in this way, It won't be observable</p>
  
  <pre><code>items.push(new SampleObject("Sample Title"));
  </code></pre>
  
  <p>How can I solve this problem !?</p>
  
  <p>What's the differences between x and y ?!</p>
  
  <pre><code>var x = {
  Title: "sample"
  }
  
  var y = new SampleObject("sample");
  </code></pre>