use readable constants for HTTP-response type In order to decide how the HTTP response of a Hets::Caller should be served, we'll use speaking constant names.

perform proposed hound-style fixes.

adjust parsing to work with io/tempfile instance In order for the API-system to work properly we'll need to work with the streamed Tempfile. So instead of working with filepaths we'll need to pass io-instances to parsing-processes.

add hets-version-caller into the mix The VersionCaller can retrieve the version of a running hets-instance by calling the restful API. It also serves as a sort of heartbeat in order to determine whether the hets-instance is reachable (internally called 'up').