enchance mapping support

src/main/java/com/alibaba/druid/mapping/Entity.java

@@ -3,7 +3,7 @@ package com.alibaba.druid.mapping;
 import java.util.LinkedHashMap;
 import java.util.Map;
 
-public class Entity {
+public class Entity extends MappingObject {
 
     private String                name;
     private String                description;

src/main/java/com/alibaba/druid/mapping/MappingObject.java

@@ -0,0 +1,37 @@
+package com.alibaba.druid.mapping;
+
+import java.util.HashMap;
+import java.util.Map;
+
+
+public class MappingObject {
+    private Map<String, Object> attributes;
+    
+    public Map<String, Object> getAttributes() {
+        if (attributes == null) {
+            attributes = new HashMap<String, Object>(1);
+        }
+
+        return attributes;
+    }
+
+    public Object getAttribute(String name) {
+        if (attributes == null) {
+            return null;
+        }
+
+        return attributes.get(name);
+    }
+
+    public void putAttribute(String name, Object value) {
+        if (attributes == null) {
+            attributes = new HashMap<String, Object>(1);
+        }
+
+        attributes.put(name, value);
+    }
+
+    public Map<String, Object> getAttributesDirect() {
+        return attributes;
+    }
+}

src/main/java/com/alibaba/druid/mapping/Property.java

@@ -1,6 +1,6 @@
 package com.alibaba.druid.mapping;
 
-public class Property {
+public class Property extends MappingObject {
     private String name;
     private String desciption;
     private String dbColumnName;