Send version string to Chronos Control

[vogti]

Add a method which can be overridden by the client to provide a string containing version information. The method should be executed periodically and the returned value submitted to Chronos Control.

public String versionString() {
    return "No version String provided";
}

This issue is related to Chronos-EaaS/Chronos-Control#25.

Implementing this feature requires unique agent identifiers. See issue #5.