Bugfix: Parameter names setup

Parameter names are left _null_ when at root level or referencing another model.

Sample code:

    <?php
    $desc = array(
       "models" => array(
          "date" => array(
             "type" => "string",
             "pattern" => "[0-9]{4}-[0-9]{2}-[0-9]{2}",
             "description" => _e("ISO 8601 date")
          ),
          "User" => array(
             "type" => "object",
             "properties" => array(
                "dob" => array('$ref' => "date")
             )
          )
       )
    );
    $sd = Guzzle\Service\Description\ServiceDescription::factory($desc);
    var_dump($sd->getModel("date")->getName());
    var_dump($sd->getModel("User")->getProperty("dob")->getName());
    die();

The sample above should display "date" and "dob", instead than _null_ and _null_.

The 2 submitted commits ensure that if no specific name is set, the parameter key is used.