更新了limits字段的表设计

Change-Id: Ieb4ad5d93ceda0b310833218432834e9e86839c4
修改案时间排序的字段有'cretime'改为'id'
 涉及到的可以修改的表：Photo，User  ，Log   ，Product  ，Statistic

Change-Id: I1eb3e5218187e00d0e2d6aac269eca4a87ad6a4b
Merge branch 'master' of https://github.com/Hahn-/GdutExchange.git