Add Message#hasAttachment convenience method

src/main/java/envoy/data/Message.java

@@ -179,6 +179,12 @@ public class Message implements Serializable {
 	 */
 	public Attachment getAttachment() { return attachment; }
 
+	/**
+	 * @return {@code true} if an attachment is present
+	 * @since Envoy Common v0.1-beta
+	 */
+	public boolean hasAttachment() { return attachment != null; }
+
 	/**
 	 * @return the current status of this message
 	 * @since Envoy Common v0.2-alpha