Feature/cancan

- url_validator.rb, fix a problem of without a validate_each funtion
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- Gemfile, install 'cancan', 'rolify'
- application_controller.rb, redirect cancan exception to root page
- controller about manage, use cancan authorization
- ability.rb, new file cancan Ability model
- role.rb, new file rolify role model
- user.rb, implement rolify
- rolify.rb, new file rolify initializer
- schema.rb, change schema when 'rake db:migrate'