v1.3.7: Add message revoke (#370)

增加了消息撤回方法，示例如下：

import itchat

itchat.auto_login(True)

user = itchat.search_friends(nickName='LittleCoder')[0]
r = user.send('hi')
msgId = r['MsgID']
r = itchat.revoke(msgId, user.userName)

itchat.run()