Moving interfaces for services to Umbraco.Core.

Adding xml extensions for Content to generate xml for the xml cache.
Adding test for xml generation.

src/Umbraco.Core/XmlExtensions.cs

@@ -1,15 +1,14 @@
 using System;
 using System.Xml;
+using System.Xml.Linq;
 
 namespace Umbraco.Core
 {
-
 	/// <summary>
 	/// Extension methods for xml objects
 	/// </summary>
 	internal static class XmlExtensions
 	{
-
 		public static T AttributeValue<T>(this XmlNode xml, string attributeName)
 		{
 			if (xml == null) throw new ArgumentNullException("xml");
@@ -26,5 +25,22 @@ namespace Umbraco.Core
 			return default(T);
 		}
 
+        public static XElement GetXElement(this XmlNode node)
+        {
+            XDocument xDoc = new XDocument();
+            using (XmlWriter xmlWriter = xDoc.CreateWriter())
+                node.WriteTo(xmlWriter);
+            return xDoc.Root;
+        }
+
+        public static XmlNode GetXmlNode(this XElement element)
+        {
+            using (XmlReader xmlReader = element.CreateReader())
+            {
+                XmlDocument xmlDoc = new XmlDocument();
+                xmlDoc.Load(xmlReader);
+                return xmlDoc;
+            }
+        }
 	}
 }