Update source guide section.
Incoming events are validated against schema.
Added TRACKERS.public field.

Field indicates that tracker is public and visible for all.
Upgrade to latest aleph 0.3.0-rc2
Support public flag for trackers
TRACKERS.public is now null, defaults to false.
Obey auth when fetching events related to trackers/sessions
Fetching sessions obeys authorization.
Expose TRACKERS.public in REST api.