title: Inject SwiftMailer into symfony2 service
body: <p>I have a service which extends UserManager, so when I do:</p>
  
  <pre><code>$message = \Swift_Message::newInstance()
              -&gt;setSubject('~')
              -&gt;setFrom('~')
              -&gt;setTo('~')
              -&gt;setBody('~', 'text/html');
          $this-&gt;get('mailer')-&gt;send($message);
  </code></pre>
  
  <p>I get the error:</p>
  
  <pre><code>Fatal error: Call to undefined method My\MyBundle\Service\ServiceClass::get()
  </code></pre>
  
  <p>I know this is because I need to inject the swiftmailer into here, but how?
  (usually the service class extends 'Generic' so the swift mailer is included.)</p>