Enhance interface for loading urdf/srdf files

  - split loadRobotModel into loadRobotModel and loadHumanoidModel,
  - above methods return a pointer to the new object.
Update documentation.
Modify interface for loading urdf/srdf files

  - Empty robot is passed to the function that parses the files. This enables
    users to create intances of derived classes and to pass them to the
    function.