amend 'spider' 'lecture' add 'activity'

1: Sometimes the code for models is too long to put into the models.py(spider, lecture for example). I'm not sure where they should locate. Now I put them in another file inside the same module.
2: Some implementation may be ugly. In the spider, when getting every announcement on www.szu.edu.cn/board/ , I have to check whether is is has been saved before, so at the same time, I have to access the database frequently. At the first time, I try to use a record to memorize the url of the message I finally get last time, so that I can judge where to stop by that url. But things goes complex because of the sticky message.