ModelSync.Multi Class
This is a class extension for Y.Module allowing multiple model sync implementations to be used by a single model. When a class is created extending Y.Model, this class extension must come after all other class extensions which implement sync. Model classes using this extension should have a static SYNCS property. The SYNCS property will be an object of key/value pairs. There should be a value for each other sync implementation extension. The keys can be any name you want to give that implementation. When calling the sync methods such as load or save, the options argument object should be given a sync property which matches the name of one of the SYNCS to use.