code: function(key, fn) {
      var self = this;
      var filepath = this._makeFilepath(key);
  
      fs.exists(filepath, function(exists) {
        if (!exists) {
          return fn();
        }
  
        fs.readFile(filepath, 'UTF8', function(err, data) {
          if (err) {
            return fn(err);
          }
  
          var cache = JSON.parse(data);
  
          if (!self._cacheIsValid(cache)) {
            return self.del(key, fn);
          }
          fn(null, cache.data);
        });
  
      });
    }
docs: /*
  @param {Mixed} key - A JSON stringifiable key that is used to lookup
  up a cached file