Restful:参考资料
http://www.infoq.com/cn/articles/designing-restful-http-apps-roth
MD5(client_id + client_secret + timestamp)
如:
String client_id = "发票云激活时提供的client_id";
String client_secret = "发票云激活时提供的client_secret";
long times = System.currentTimeMillis();
String checkSign = MD5.md5crypt(client_id + client_secret + timestamp);