Modified player registration endpoints

The endpoints now all accept and return the same kind of JSON data.
Available endpoints are:

POST   /player         Register a new player
GET    /player         Get information about all players
GET    /player/<name>  Get information about player <name>
DELETE /player/<name>  Delete player

To register a player, you can run:

  curl -i --request POST -H 'Content-Type: application/json' \
    --data '{"name":"player01","description":"The first player."}' \
    "http://127.0.0.1:5001/player"

Thus the data you need to send are:

  {
    "name":"player01",
    "description":"The first player."
  }

The same kind of data are returned by the GET methods:

  % curl http://127.0.0.1:5001/player
  {
    "player": [
      {
        "current": null,
        "description": "The first player.",
        "name": "player01"
      }
    ]
  }

  % curl http://127.0.0.1:5001/player/player01
  {
    "current": null,
    "description": "The first player.",
    "name": "player01"
  }

Signed-off-by: Lars Kiesow <lkiesow@uos.de>