title: Symfony Assert\Expression compare entity property with today's date
body: <p>I'd like to add an Assert\Expression to a property of my entity.
  There should be an error message popping up, when the selected "Effective date" (this.getEffective()) is in the past or today. I just can't figure out, how to compare that date with todays date. </p>
  
  <pre><code>    /**
   * @var boolean
   * @ORM\Column(type="boolean", nullable=true)
   * @Assert\Expression(
   *     "this.getEffective() &gt; today",
   *     message="The effective date must be in the future!")
   */
  private $status_stealth;
  </code></pre>
  
  <p>I tried the same with <strong>now</strong> or <strong>datetime.now()</strong> and I've google around a lot, but I haven't found anything where somebody actually compared another value with the current date within an Assert Expression.</p>
  
  <p>Ideas?</p>