Add EXP. to problem
New model and controller for site settings
Manage MAX_LEVEL by Setting
Add edit setting page
Mange announcement by Setting
Add helper functions of Setting
Add EXP. to user
Add setting of MAX_DIFFICULTY
Create, update and show exp of problem(s)
Init level exp settings after change max level and update them after create/edit program
Show exp info in current level page
fix : let admin see hidden problems
set exp default to 1 in problems table
add an edit option in problem list for admin
Primitive exp-based level system
Merge pull request #2 from AlphardWang/development

fix : let admin see hidden problems
Change user exp after edit problem exp
Filter hidden problems' submissions from submission index
Merge branch 'development'