Update index.js

Add count function (2)
My propose: 

  this.count = function (model,callback) {
    var collectionName = _collectionizeModelName(model.modelName)
      , collection = this.client.collection(collectionName)
      ;

    collection.count(function (err,data){
      if (data) {
        callback(null,data);
      } else if (err) {
        callback(err,null);
      }
    });
  }