Merge pull request #108 from tritemio/model_noparams

Fix typo Thanks!!
parameter can have max but no value and no min.
added parameter hints to Model: _param_hints and _set_param_hints() to set default vals/attributes
updated subclasssed Models to use Model._set_param_hint
better syntax for Model.set_param_hint()
add mailing list / issue track links to doc sidebar
allow prefix or not in model.set_param_hint()
Rectangle model -- midpoint set with set_param_hint()