Select: ensure the API of delite/Selection is always called for data items, not for StoreMap's render items; fix of getIdentity() to take into account store's idProperty. Refs #255

This takes profit from the recent https://github.com/cjolif/delite/commit/247229c283c69606c08aacdaba2470e475bf6e26. Hence, it MUST not be merged before this commit will be included in a new release of delite used by deliteful.
isn't the data item in `__item` not `_dataItem`?