Add SetAirplayVolume to the D-Bus RemoteControl interface.

Add SetVolume to the MPRIS interface.
Hook up the Volume property in the MPRIS interface.
Modify RemoteCommand in the D-Bus interface to return the HTTP status and response.
Change the type of airplay_volume from int to double in the metadata hub.
Add a few sample commands in the D-Bus document.

org.gnome.ShairportSync.xml

@@ -13,6 +13,8 @@
     <property name="DriftTolerance" type="d" access="readwrite" />
     <method name="RemoteCommand">
       <arg name="command" type="s" direction="in" />
+      <arg name="reply" type="i" direction="out" />      
+      <arg name="response" type="s" direction="out" />
     </method>
     <property name="VolumeControlProfile" type="s" access="readwrite" />
     <property name="Interpolation" type="s" access="readwrite" />
@@ -45,6 +47,9 @@
 		<property name='ProgressString' type='s' access='read'/>
 		<property name='Server' type='s' access='read'/>
 		<property name='AirplayVolume' type='d' access='read'/>
+    <method name="SetAirplayVolume">
+      <arg name="volume" type="d" direction="in" />
+    </method>
 		<property name='Metadata' type='a{sv}' access='read'>
 			<annotation name="org.qtproject.QtDBus.QtTypeName" value="QVariantMap"/>
 		</property>