Implement Index API

Three commits here:
1. Fixed the ObjectStore so that it finds existing indexNames when the store is created.
2. Create EIDB.Index. Deviates from the published API by returning 'event.target.result' instead of the event (as ObjectStore#get does.) ObjectStore returns this instead of IDBIndex.
3. Added a non-published API method Index#getAll. Seems that this would be useful.