Add communication semaphore class

[harrydehix]

This class is an abstraction for http requests to the backend. Every response is typesafe!
Changes in the backend have to get reflected in the implementation of this class.
Example usage:

API.initialize("localhost:3000");
const weatherStations = await API.getAvailableWeatherStations();
const currentSensorData = await API.getCurrentSensorValues(globals.selectedWeatherStation.slug);
const sessionToken = await API.adminLogin(username, password);
...

[harrydehix]

At first there should be a dummy implementation which returns fixed values.