Support sharding information in CollStats command

[sgodbillon]

Example:

{
  sharded: BSONBoolean(true),
  ns: BSONString(central.recIndex),
  count: BSONInteger(18485538),
  numExtents: BSONInteger(66),
  size: BSONInteger(13479700),
  storageSize: BSONInteger(15785124),
  totalIndexSize: BSONInteger(5412934),
  indexSizes: {
    _id_: BSONInteger(931257),
    extno_1: BSONInteger(773173),
    serverid_1: BSONInteger(525338),
    siteid_1: BSONInteger(525338),
    siteid_1_serverid_1_weekOfYear_1_extno_1: BSONInteger(1768705),
    weekOfYear_1: BSONInteger(889114)
  },
  avgObjSize: BSONDouble(0.7292024716835399),
  nindexes: BSONInteger(6),
  nchunks: BSONInteger(136),
  shards: {
    shard0000: {
      ns: BSONString(central.recIndex),
      count: BSONInteger(6127876),
      size: BSONInteger(4469920),
      avgObjSize: BSONDouble(0.7294403476832756),
      storageSize: BSONInteger(5261708),
      numExtents: BSONInteger(22),
      nindexes: BSONInteger(6),
      lastExtentSize: BSONInteger(1371536),
      paddingFactor: BSONDouble(1.0),
      systemFlags: BSONInteger(1),
      userFlags: BSONInteger(0),
      totalIndexSize: BSONInteger(1805155),
      indexSizes: {
        _id_: BSONInteger(310664),
        siteid_1: BSONInteger(176374),
        serverid_1: BSONInteger(176374),
        weekOfYear_1: BSONInteger(296938),
        extno_1: BSONInteger(258326),
        siteid_1_serverid_1_weekOfYear_1_extno_1: BSONInteger(586476)
      },
      ok: BSONDouble(1.0)
    },
    shard0001: {
      ns: BSONString(central.recIndex),
      count: BSONInteger(6038599),
      size: BSONInteger(4398113),
      avgObjSize: BSONDouble(0.7283333435454151),
      storageSize: BSONInteger(5261708),
      numExtents: BSONInteger(22),
      nindexes: BSONInteger(6),
      lastExtentSize: BSONInteger(1371536),
      paddingFactor: BSONDouble(1.0),
      systemFlags: BSONInteger(1),
      userFlags: BSONInteger(0),
      totalIndexSize: BSONInteger(1762375),
      indexSizes: {
        _id_: BSONInteger(310927),
        siteid_1: BSONInteger(164565),
        serverid_1: BSONInteger(164565),
        weekOfYear_1: BSONInteger(290064),
        extno_1: BSONInteger(255092),
        siteid_1_serverid_1_weekOfYear_1_extno_1: BSONInteger(577158)
      },
      ok: BSONDouble(1.0)
    },
    shard0002: {
      ns: BSONString(central.recIndex),
      count: BSONInteger(6319063),
      size: BSONInteger(4611667),
      avgObjSize: BSONDouble(0.729802345695873),
      storageSize: BSONInteger(5261708),
      numExtents: BSONInteger(22),
      nindexes: BSONInteger(6),
      lastExtentSize: BSONInteger(1371536),
      paddingFactor: BSONDouble(1.0),
      systemFlags: BSONInteger(1),
      userFlags: BSONInteger(0),
      totalIndexSize: BSONInteger(1845404),
      indexSizes: {
        _id_: BSONInteger(309666),
        siteid_1: BSONInteger(184399),
        serverid_1: BSONInteger(184399),
        weekOfYear_1: BSONInteger(302112),
        extno_1: BSONInteger(259755),
        siteid_1_serverid_1_weekOfYear_1_extno_1: BSONInteger(605071)
      },
      ok: BSONDouble(1.0)
    }
  },
  ok: BSONDouble(1.0)
}

(See Play-ReactiveMongo #83)