Store sync events

[ExtKernel]

Store sync history/events. It'd be great to store a difference map in the event object.

The object may look like:

• Name: implement some convention for the name
• Timestamp
• Difference map
• Target object type: user or usergroup
• Status
• Occurred exceptions

[ExtKernel]

Partially done in the commit 1217052

Exceptions are not recorded yet.