loading-time parent store references (singleton mixins)

[avnil007]

While defining the store, provide direct reference to the parent store object. In Store.define(), extract parent store type, and mix in its attributes. This logic (singleton mixins) can be done for regular models too.

Currently, it can be done specifying _owner in store definition, but it doesn't mix in owner's properties. This feature is important for ad-hoc per page stores.