Introduce audit trail functionality

Draft implementation of audit trail support, to notify something/someone
that actions are being taken against an environment by Maestro. Supports
HipChat (via python-simple-hipchat) and log file for now.

Signed-off-by: Maxime Petazzoni <max@signalfuse.com>
Wording fix in time_ago

Signed-off-by: Maxime Petazzoni <max@signalfuse.com>