title: Find tag by id using acts_as_taggable_on
body: <p>I'm using Ruby on rails 2.3.8 and acts_as_taggable_on plugin. This plugin generates three tables: tags(which has the id and name of each tag) and taggings(relates tag_ids which the items that are tagged).</p>
  
  <p>Now, I'd like to get all the items tagged with a certain tag_id. So, I go to my Announcement model(the announcements are the tagged items) and I'd like to write something like this:</p>
  
  <pre><code>def find_by_category(tag_id)
    Announcement.tagged_with(tag_id) #this doesn't exist.
  end
  </code></pre>
  
  <p>But the only method I found actually was this:</p>
  
  <pre><code>Announcement.tagged_with(tag_NAME) #this works, but I don't want to find by a name.
  </code></pre>
  
  <p>Does anybody know a method to accomplish this, or at least, how to modify the plugin's code to do this?</p>