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/Property.cs

@@ -61,6 +61,12 @@ namespace Umbraco.Core.Models
         [IgnoreDataMember]
         internal DataTypeDatabaseType DataTypeDatabaseType { get { return _propertyType.DataTypeDatabaseType; } }
 
+        /// <summary>
+        /// Returns the PropertyType, which this Property is based on
+        /// </summary>
+        [IgnoreDataMember]
+        internal PropertyType PropertyType { get { return _propertyType; } }
+        
         /// <summary>
         /// Gets or Sets the version id for the Property
         /// </summary>