Split pages into admins' pages and contestants' pages

Refer to #71 

There are many changes in this requrst, so that please review carefully.

Contestants' routes
--------
name                         |method| route                                                     | controller#action
------------------------------|------|-----------------------------------------------------------|--------------------------
                      contests|GET   |/contests(.:format)                                        |contests#index
                       contest|GET   |/contests/:id(.:format)                                    |contests#show
                   login_users|GET   |/users/login(.:format)                                     |users#login
                register_users|GET   |/users/register(.:format)                                  |users#register
               authorize_users|POST  |/users/authorize(.:format)                                 |users#authorize
                  logout_users|POST  |/users/logout(.:format)                                    |users#logout
                         users|POST  |/users(.:format)                                           |users#create
                     edit_user|GET   |/users/:id/edit(.:format)                                  |users#edit
                          user|GET   |/users/:id(.:format)                                       |users#show
                              |PUT   |/users/:id(.:format)                                       |users#update
                              |DELETE|/users/:id(.:format)                                       |users#destroy

Administrators' routes
--------
name                         |method| route                                                     | controller#action
------------------------------|------|-----------------------------------------------------------|--------------------------
                   admin_users|GET   |/admin/users(.:format)                                     |admin/users#index
                              |POST  |/admin/users(.:format)                                     |admin/users#create
                new_admin_user|GET   |/admin/users/new(.:format)                                 |admin/users#new
               edit_admin_user|GET   |/admin/users/:id/edit(.:format)                            |admin/users#edit
                    admin_user|GET   |/admin/users/:id(.:format)                                 |admin/users#show
                              |PUT   |/admin/users/:id(.:format)                                 |admin/users#update
                              |DELETE|/admin/users/:id(.:format)                                 |admin/users#destroy
        admin_contest_problems|GET   |/admin/contests/:contest_id/problems(.:format)             |admin/contests/problems#index
                              |POST  |/admin/contests/:contest_id/problems(.:format)             |admin/contests/problems#create
     new_admin_contest_problem|GET   |/admin/contests/:contest_id/problems/new(.:format)         |admin/contests/problems#new
    edit_admin_contest_problem|GET   |/admin/contests/:contest_id/problems/:id/edit(.:format)    |admin/contests/problems#edit
         admin_contest_problem|GET   |/admin/contests/:contest_id/problems/:id(.:format)         |admin/contests/problems#show
                              |PUT   |/admin/contests/:contest_id/problems/:id(.:format)         |admin/contests/problems#update
                              |DELETE|/admin/contests/:contest_id/problems/:id(.:format)         |admin/contests/problems#destroy
                admin_contests|GET   |/admin/contests(.:format)                                  |admin/contests#index
                              |POST  |/admin/contests(.:format)                                  |admin/contests#create
             new_admin_contest|GET   |/admin/contests/new(.:format)                              |admin/contests#new
            edit_admin_contest|GET   |/admin/contests/:id/edit(.:format)                         |admin/contests#edit
                 admin_contest|GET   |/admin/contests/:id(.:format)                              |admin/contests#show
                              |PUT   |/admin/contests/:id(.:format)                              |admin/contests#update
                              |DELETE|/admin/contests/:id(.:format)                              |admin/contests#destroy