Make the Messenger class Abstract
Add the User Agent class

This class contains several static user agent strings.
Update the web client class

Modify:
Make the class abstract;
Change the static method "sendGet/sendPost" to dynamic, deal the
Exceptions inner the method, remove save methods to "saveRspToFile", and
record the last response for further use;
Add attribute lastRsp to save the last response, and add its get method;
Add "show LastResponseSummary" method to To show status line and headers
of the last response;
Add "useEasyCookiePolicy" and "setUserAgent" methods to simplized
operations.