revise stateGet rest endpoint

[pbelmann]

The following points must be revised:

• decide and document if stderr and stdout are paths to a file or should contain the output of the corresponding files.
• code should be an int
• since each framework reports to a different degree the exit code of the process we should clarify that at least the succesful exit of a job should be 0. Any other exit code determined by the framework is up to the jobproxy implementation
• the stateGet rest endpoint returns an object called States that contains a list of state objects. In my opinion we can skip this object and simply returns a list of state objects
• How about adding a status field? It should return running or finished.
• Rename rest method statePost to specificState because statePost is not using post.