[FEATURE] introduce http_utils class

to format responses
[FEATURE] convert info class to use http_utils to respond
[TASK] rename express_app to app
[TASK] use http_utils for respond in app config

- format
- comment
- clean up includes
[TEST] add postman collection
[FEATURE] add bad request to http_utils

use message as default property for error-like responses
[TASK] remove unused require
[FEATURE] convert balances to use http_utils for responses
[TASK] rename http_utils to response-handler
[FEATURE] lay out structure for error codes and responses

all responses should go through the response-handler
the response-handler manages types, status codes and success flags
available methods are exported and should be used by the controllers
[TASK] adopt response-handler methods and cleanup
[FEATURE] use custom Error classes for various error types
Merge pull request #125 from geertweening/develop

WIP: rework response handling