规范package，为发布到maven central repository做准备

src/main/java/me/chanjar/weixin/util/xml/MediaIdMarshaller.java

@@ -0,0 +1,21 @@
+package me.chanjar.weixin.util.xml;
+
+import javax.xml.bind.annotation.adapters.XmlAdapter;
+
+/**
+ * @author chanjarster
+ */
+public class MediaIdMarshaller extends XmlAdapter<String, String> {
+
+  @Override
+  public String marshal(String arg0) throws Exception {
+    return "<MediaId><![CDATA[" + arg0 + "]]></MediaId>";
+  }
+
+  @Override
+  public String unmarshal(String arg0) throws Exception {
+    // do nothing
+    return arg0;
+  }
+
+}