Improve : Make the production service URL the default one.
Problem : The default service URL was pointing to the test server(https://api.korbit.co.kr:8080/) and it makes impossible to use production API server.
Solution : Change the default URL prefix to point to the production server(https://api.korbit.co.kr) and allow to override it by setting URLPrefix.prefix var.
Additional Fix : Added type annotation for the returned type in callbacks to make the sample code more readable.
Merge branch 'master' of https://github.com/Kangmo/korbit-scala-sdk