Refactoring ExpressionHelpers and querying to seperate the mapping used for our Database DTOs and Public Model.

Adding a MediaTypeMapper similar to the ContentTypeMapper, but type specific.
The PetaPoco IMapper implementation is removed as its no longer needed.
Updating a few unit tests, as the Where clause is now correctly formatted.
Adding unit tests for the new ExpressionHelper and MappingResolver classes.

src/Umbraco.Core/Persistence/PetaPocoSqlExtensions.cs

@@ -22,7 +22,7 @@ namespace Umbraco.Core.Persistence
 
         public static Sql Where<T>(this Sql sql, Expression<Func<T, bool>> predicate)
         {
-            var expresionist = new ExpressionHelper<T>();
+            var expresionist = new PocoToSqlExpressionHelper<T>();
             string whereExpression = expresionist.Visit(predicate);
 
             return sql.Where(whereExpression);