fix #48: notification refresh
* Store notification cache in sqlite
* Fetch new notifications when a push is received
* Broadcast a "refresh-me-please" message when a push is received so NotificationsActivity is refreshed if it's running and a push is received
* read and unzip full_note_data in payload if exists and add it to the notification cache (props to @daniloercoli for idea and code)
* if full_note_data is not in the payload, a temporary placeholder is created. Placeholders are deleted when notifications are refreshed with a REST call.
change timestamp field name from "timestamp" to "note_timestamp"
Merge branch 'fix-48' into develop