api,dbus: new `Command' method in the API to send arbitrary AT commands

BUG=chromium-os:25348
TEST="mmcli -m 0 --command='E0'"
Change-Id: I320587560fde5780f9d5a4998e32364d36a71ed7

new/org.freedesktop.ModemManager1.Modem.xml

@@ -146,6 +146,20 @@
       <arg name="bands" type="au" direction="in" />
     </method>
 
+    <!--
+       Command
+       @cmd The command string, e.g. "AT+GCAP" or "+GCAP" (leading AT is inserted if necessary)
+       @timeout The number of seconds to wait for a response
+       @response The modem's response
+
+       Send an arbitrary AT command to a modem and get the response.
+      -->
+    <method name="Command">
+      <arg name="cmd"      type="s" direction="in"  />
+      <arg name="timeout"  type="u" direction="in"  />
+      <arg name="response" type="s" direction="out" />
+    </method>
+
     <!--
         StateChanged:
         @old: A <link linkend="MMModemState">MMModemState</link> value, specifying the new state.