Adapt client to the server

Refactor HarvestJournalEntry into a public class HarvestEntry.
The idea is to use this HarvestEntry to handle the interaction
between all classes.

Modify serialization format to adapt it to the back-end, the idea
is to save redundant information while transmitting the report.
This is achieved by indexing the apps usage sesions by package
name.

Rename SQLite DB to match server naming conventions, and finally,
do a generate code cleanup.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Temporarily reduce persist and report intervals

There is still missing a proper logic for controlling
the report intervals, ie., persisting lastReported, that
are necessary to implement weekly reports.

This is change is for mere developing purposes.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Add TODO

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Update README

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>