参考资料：
1）Hibernate复合主键映射 1 http://wpmqd.blog.163.com/blog/static/7730899320121111802450/
2）Hibernate中复合主键的映射 http://sarin.iteye.com/blog/644678
3）Hibernate 复合主键http://zhangli-lisa.iteye.com/blog/476974