title: How to disable a button when the form is invalid?
body: <p>I have a custom element that basically contains an email and password form, </p>
  
  <p>Is there any way to disable the button when the email or the password are not filled ?</p>
  
  <p>Please find below my element:</p>
  
  <pre><code>      &lt;paper-dialog id="dialog"&gt;
      &lt;h2&gt;Login&lt;/h2&gt;
  
        &lt;paper-input id="zumbuza" type="email" is="iron-input" label="Email" type="text" bind-value="{{user.email}}" auto-validate  error-message="Auto v"&gt;&lt;/paper-input&gt;
  
  
      &lt;paper-input-container&gt;
         &lt;label&gt;Passwrod&lt;/label&gt;
         &lt;input  is="iron-input" type="password" bind-value="{{user.password}}"&gt;&lt;/input&gt;
      &lt;/paper-input-container&gt;
  
      &lt;paper-button toggles raised active class="colorful"&gt;Submit&lt;/paper-button&gt;
  
    &lt;/paper-dialog&gt;
  </code></pre>