shill: Provide notification callbacks on DHCP configuration updates.

BUG=chromium-os:16257
TEST=unit tests

Change-Id: Ifef3eb4e295bea21a05bce4af440536472953a8b
Reviewed-on: http://gerrit.chromium.org/gerrit/2273
Tested-by: Darin Petkov <petkov@chromium.org>
Reviewed-by: Chris Masone <cmasone@chromium.org>
3 files changed