Loads the cached database

@param callback
function(callback) {
      var t = this;
      async.waterfall([
        function readDb(callback) {
          fs.exists(t.__dbFilename, function(exists) {
            if (exists) {
              fs.readFile(t.__dbFilename, {
                encoding: "utf-8"
              }, callback);
            } else {
              callback(null, null); 
            }
          });
        },

        function parseDb(data, callback) {
          t.__db = data && data.trim().length ? jsonlint.parse(data) : {};
          callback(null, t.__db);
        }
      ], callback);
    }