title: swagger-codegen simple models missing
body: <p>I'm realizing my second API with swagger/swagger-codegen.
  After having a really good start with my first one I'm somewhat stuck with the following problem:
  I'm having multiple definitions like the following:</p>
  
  <pre><code>TopIssueReference:
    description: Id of a top issue
    type: string
    example:
      itemid: 'd32c1213-4773-442e-9c5f-f5d516358869'
  </code></pre>
  
  <p>All those definitions only are aliases for type string, some with format date-time, some naked like the one above.</p>
  
  <p>The swagger editor is fine with those definitions.</p>
  
  <p>When I use one of them in a <code>$ref</code> clause within some object definition the generator produces a reference to a class named like my definition, <code>TopIssueReference</code> in this case.</p>
  
  <p>The generated <code>TopIssueReference</code> class is the following (in java):</p>
  
  <pre><code>@ApiModel(description = "Id of a top issue")
  public class TopIssueReference  {
    @Override
    public String toString()  {
      StringBuilder sb = new StringBuilder();
      sb.append("class TopIssueReference {");
      sb.append("}");
      return sb.toString();
    }
  }
  </code></pre>
  
  <p>which is not really useful.</p>
  
  <p>Does anybody have an idea what's going wrong here?
  Shouldn't the generator either produce a reference to a String or at least make <code>TopIssueReference</code> derive from a string (however useful that may be)?</p>
  
  <p>For some of those definitions, the generator does not generate any class at all - but the references are still there, so the resulting code does not even compile.</p>
  
  <p>I tried generating servers and clients with java and python, and both are having the same problem.</p>