new PID api.

HTTP PID api is now :

    GET /<pid> : return info of the Process with id <pid>
    HEAD /<pid>: tell the user if the Process ID is active
    DELETE /<pid>: stop the Process <pid>

    GET /<pid>/stats: return stats of the process
    POST /<pid>/signal {"signal": str or int}Â : send a signal to the process

To get a `gaffer.Process` instance you should now use `Manager.get_pid` and to
stop id use `Manager.stop_pid` .

`Process.info` now only return the process metadata (pid, os pid, name, cmd,
...). To get the stats of a `Process` instance you should now use the method
`Process.stats`.