title: Type error when trying to use mapped projection with Slick
body: <p>I'm trying to use Slick's mapped projections (Version 1.0.0-RC1). But the following code, which follows the examples on the website (as there doesn't seem to be any proper documentation nor usable scaladocs) yields a type error:</p>
  
  <pre><code>object PDFDocs extends Table[(String,Option[String],String)]("DOCUMENTS"){
    def id = column[String]("ID", O.PrimaryKey)
    def title = column[Option[String]]("TITLE")
    def tags = column[String]("TAGS")
    def * = (id ~ title ~ tags).&lt;&gt;[PDFDocument](PDFDocument,PDFDocument unapply _)
  }
  
  case class PDFDocument(name: String,
                         title: Option[String],
                         tags: String)
  </code></pre>
  
  <p>Here is the produced error:</p>
  
  <pre><code>error: type mismatch; 
  found: scala.slick.lifted.MappedProjection[docman.rdb.PDFDocument,(String,Option[String], String)] 
  required: scala.slick.lifted.ColumnBase[(String, Option[String], String)] 
  def * = (id ~ title ~ tags).&lt;&gt;[PDFDocument](PDFDocument,PDFDocument unapply _)
  </code></pre>