rework Store architecture based on SitePen/dstore#5

This PR is changing a bit the design of delite/Store to comply with what was mentionned in SitePen/dstore#5.

In particular:
- range should always be called post track
- notification can come from outside the range

For that postProcessStore is replaced by start & end range index and the notification mechanism is checking if the notification comes from items inside the range before:

1. continuing with notification only if the change is in the range
1. changing the notification index to be based on the range start index

I'm particularly interested in @sbrunot feedback on whether this would be fine with `delite/list/List`?