AU: DBus support.

A few changes to support dbus in the Update Engine daemon:

- SConstruct: build marshaller for the dbus signal.

- Update Attempter: respond to dbus calls and broadcast status on dbus
  signal.

- Update Engine Client: flag to listen for status updates.

- Also, cleanup outdated code in Omaha Response Handler.

BUG=None
TEST=attached unittests/on device tests

Review URL: http://codereview.chromium.org/2037002
diff --git a/UpdateEngine.conf b/UpdateEngine.conf
index 88d6dc4..fe332c0 100644
--- a/UpdateEngine.conf
+++ b/UpdateEngine.conf
@@ -16,6 +16,9 @@
     <allow send_destination="org.chromium.UpdateEngine"
            send_interface="org.chromium.UpdateEngineInterface"
            send_member="GetStatus"/>
+    <allow send_destination="org.chromium.UpdateEngine"
+           send_interface="org.chromium.UpdateEngineInterface"
+           send_member="CheckForUpdate"/>
   </policy>
   <policy context="default">
     <deny send_destination="org.chromium.UpdateEngine" />