Added rng for external images

server/rngs/externalImage.rng

@@ -0,0 +1,13 @@
+<grammar xmlns="http://relaxng.org/ns/structure/1.0" datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">
+  
+  <define name="externalImage">
+    <element name="externalImage">
+      <element name="url">
+        <text/>
+      </element>    
+      <element name="lang">
+        <text/>
+      </element>     
+    </element>
+  </define>
+</grammar>

server/rngs/t_externalImage.rng

@@ -0,0 +1,7 @@
+<grammar xmlns="http://relaxng.org/ns/structure/1.0" datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">
+  <include href="externalImage.rng"/>
+
+  <start>
+    <ref name="externalImage"/>
+  </start>
+</grammar>