Do some Refactoring.

- 用ItemType type 取代 int topicType ,程式碼會比較好懂.
詳情看 ExamItemAdd.java 的diff會比較清楚
#39 Multi choose 出題OK
#39 新增填充題新增方法
Refactoring for ItemType.

Used simple factory pattern & strategy pattern.
Merge pull request #40 from MilesChou/39-multi-n-fill

#39