cashew: Add ipconfig dbus object

Add ipconfig dbus object

BUG=chromium-os:27904
TEST=fetch usage information while attached to Ethernet

Change-Id: I112f6c1e07657215170c925d06a6bde245186d47
Reviewed-on: https://gerrit.chromium.org/gerrit/18802
Reviewed-by: Nathan J. Williams <njw@chromium.org>
Commit-Ready: <jglasgow@google.com>
Tested-by: <jglasgow@google.com>