Proxy Model & ModelTreeManager Enhancement

ModelTreeManager can now be initialized with a specific tree rather than assume a tree given a model (since a model could have multiple trees associated with it.)

A new optional argument model has been added for query_string_for_field and query_condition for specifying the model the field is coming from. This is specifically to support proxy models in Django which allows for defining independent, logical models on an existing one. ModelTree can take advantage of this since different join paths will be inferred.