Rename: app.rb -> server.rb
Introduce Restangular

"AngularJS service to handle Rest API Restful Resources properly and
easily"
Implement GET behavior of Restangular

Reference: https://github.com/mgonto/restangular#lets-code
Rewrite DB related codes of server.rb

1. Sequel::Model => Sequel::Dataset
2. Now server.rb receives client data as json