title: collationStringSelector error : Unrecognized selector
body: <p>I am trying to implement alphabetical sections in my application to learn Swift.
  I found this tutorial to help me : <a href="http://www.pumpmybicep.com/2014/07/04/uitableview-sectioning-and-indexing/" rel="nofollow">http://www.pumpmybicep.com/2014/07/04/uitableview-sectioning-and-indexing/</a></p>
  
  <p>First, I separate the class from the UITableViewController class for more readability. </p>
  
  <p>But I have an error on the collationStringCollector, it does not recognize "m_titre" and I do not understand how collationStringSelector works and so, how to use it.</p>
  
  <pre><code>var lesCours: [Cours] = cours_array.map { eachCours in // cours_array contain all "Cours" which will be indexed
          eachCours.section = self.collation.sectionForObject(eachCours, collationStringSelector: "title")
          return eachCours
      }
  </code></pre>
  
  <p>Here is my class "Cours"</p>
  
  <pre><code>class Cours {
  var m_id : String
  var m_titre : String
  var m_content : String
  var m_accept : Bool
  var m_date : NSDate?
  var section: Int?
  
  init(titre:String, content:String)
  {
      self.m_titre = titre
      self.m_content = content
      self.m_id = ""
      self.m_accept = false
      self.m_date = nil
  }
  
  // ... getters, setters, specifics function
  </code></pre>
  
  <p>Thank you</p>
  
  <p>PS : I apologize if my English is not perfect, it's not my native language, I hope that you will understand without difficulty in despite of this</p>