Fix model retrival from store

*Bug*: fetch would create a new model on page change - this would override the cache, killing the model stored in the cache.  This destroys the models - but leaves some listeners around, cause memory leaks (that's bad too).

*The fix*: Checks to see if it's already a model, if it is, just return that instead of creating a new model (where the initialize method calls .store to save the model in the modelStore).
Seems like you can remove `modelOptions` as a parameter now.