code: function putCache(key, contents, callback) {
      key = key.replace(/\//g, '-');
      var cacheFile = __dirname + '/' + CACHE_PREFIX + key + '.cache';
  
      fs.writeFile(cacheFile, contents, function (err) {
          if (err) {
              console.error('Error adding response to cache at ' + cacheFile);
              throw err;
          } else if (callback) {
              callback();
              return;
          }
      });
  }
docs: /*
  Put an item in the cache
  
  @param string key
  @param JSON contents
  @param Function callback (optional)
  @invoke callback()