factor out D-Bus notification to a python script

This patch is a part of our effort to move application logic from
configuration files to executables.

Related to #684

Signed-off-by: Jakub Filak <jfilak@redhat.com>
spec: add abrt-action-notify

Related to #684

Signed-off-by: Jakub Filak <jfilak@redhat.com>
DBus Problems API: update documentation

Related to #684

Signed-off-by: Jakub Filak <jfilak@redhat.com>