修改javadoc, readme, 单元测试代码

src/test/java/chanjarster/weixin/api/WxMediaAPITest.java

@@ -1,5 +1,6 @@
 package chanjarster.weixin.api;
 
+import java.io.File;
 import java.io.IOException;
 import java.io.InputStream;
 import java.util.ArrayList;
@@ -57,7 +58,8 @@ public class WxMediaAPITest {
   
   @Test(dependsOnMethods = { "testUploadMedia" }, dataProvider="downloadMedia")
   public void testDownloadMedia(String media_id) throws WxErrorException {
-    wxService.mediaDownload(media_id);
+    File file = wxService.mediaDownload(media_id);
+    Assert.assertNotNull(file);
   }
   
   @DataProvider