Per-item actions for timeshift

- Adds a generic ability to augment the presentation actions menu with
item_type-specific actions.
- Adds specific items to change the interval on
`timeshift_summation_table` (issue #217)
- Dashboard item descriptors passed to `ds.register_dashboard_item()`
can now have an `actions` attribute with the custom actions
-
Merge pull request #235 from urbanairship/issue-217-actions-per-item-type

Per-item actions for timeshift