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- Also, cleanup outdated code in Omaha Response Handler.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/update_engine.xml b/update_engine.xml
index 11c804f..f5faaeb 100644
--- a/update_engine.xml
+++ b/update_engine.xml
@@ -14,5 +14,14 @@
       <arg type="s" name="new_version" direction="out" />
       <arg type="x" name="new_size" direction="out" />
     </method>
+    <method name="CheckForUpdate">
+    </method>
+    <signal name="StatusUpdate">
+      <arg type="x" name="last_checked_time" />
+      <arg type="d" name="progress" />
+      <arg type="s" name="current_operation" />
+      <arg type="s" name="new_version" />
+      <arg type="x" name="new_size" />
+    </signal>
   </interface>
 </node>