add signature check

src/main/java/chanjarster/weixin/api/WxService.java

@@ -9,6 +9,18 @@ import chanjarster.weixin.exception.WxErrorException;
  */
 public interface WxService {
   
+  /**
+   * <pre>
+   * 验证推送过来的消息的正确性
+   * 详情请见: http://mp.weixin.qq.com/wiki/index.php?title=接入指南
+   * </pre>
+   * @param timestamp
+   * @param nonce
+   * @param signature
+   * @return
+   */
+  public boolean checkSignature(String timestamp, String nonce, String signature);
+  
   /**
    * <pre>
    * 获取access_token，本方法线程安全