mapstore - default new json etent

[gaetanbrl]

(really minor issue)

The datadir new.json extent is center on a specific region.
It could be nice to use a more largest area instead :

    "maxExtent": [
      -1076658.754450426,
      5134999.16310118,
      2166310.6398374303,
      6771685.279280833
    ],