完成数据库模块(未测试)
暴露接口给外部(DatabaseHelper类的公共方法)
I'm dying

Signed-off-by: Harry Chen <373685963@qq.com>
Merge branch 'dao' of https://github.com/PaperAirplane-Dev-Team/Mistake_Collection-Android into dao

# By xavieryao
# Via xavieryao
* 'dao' of https://github.com/PaperAirplane-Dev-Team/Mistake_Collection-Android:
  FragmentMistakes的分类列表 啊啊啊做不出设计图的效果…… Signed-off-by: xavieryao <xavieryao@live.cn>