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/Models/PropertyExtensions.cs

@@ -0,0 +1,26 @@
+using System;
+using System.Xml;
+using System.Xml.Linq;
+
+namespace Umbraco.Core.Models
+{
+    public static class PropertyExtensions
+    {
+        /// <summary>
+        /// Creates the xml representation for the <see cref="Property"/> object
+        /// </summary>
+        /// <param name="property"><see cref="Property"/> to generate xml for</param>
+        /// <returns>Xml of the property and its value</returns>
+        public static XElement ToXml(this Property property)
+        {
+            string nodeName = property.Alias.ToUmbracoAlias();
+
+            var xd = new XmlDocument();
+            XmlNode xmlNode = xd.CreateNode(XmlNodeType.Element, nodeName, "");
+            xmlNode.AppendChild(property.PropertyType.DataType(property.Id).Data.ToXMl(xd));
+
+            var element = xmlNode.GetXElement();
+            return element;
+        }
+    }
+}