title: user vaidation / unique email using cartylyst sentinel
body: <p>I want to validate user using sentinel and i want to check that unique user email_address my post request looks like:</p>
  
  <pre><code> public function postRegister(Request $request)
  {
      $user = Sentinel::register($request-&gt;all());
      $activation = Activation::create($user);
  
      $this-&gt;sendEmail($user, $activation-&gt;code);
      return redirect()-&gt;back()-&gt;with([
          'sucess' =&gt; 'user registered successfully'
      ]);
  }
  </code></pre>
  
  <p>Here i am adding first_name , last_name , email_address and password but i want to validate these fields and with unique email_address?
  How i can do that?
  Your help will be highly appreciated!</p>