#40 fix circular module dependence

src/message-media.ts

@@ -75,3 +75,4 @@ class MediaMessage extends Message {
 }
 
 export default MediaMessage
+export { MediaMessage }

src/message.ts

@@ -289,13 +289,8 @@ class Message {
 Message.initType()
 
 export default Message
-
-import MediaMessage from './message-media'
-
-export {
-    Message
-  , MediaMessage
-}
+export { Message }
+export { MediaMessage } from './message-media'
 
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