Storage main interface constructor
@memberof TableExport.prototype
@constructor
function() {
    this._instance = null;

    this.store = sessionStorage;
    this.namespace = TableExport.prototype.defaultNamespace;
    this.getKey = function(key) {
      return this.namespace + key;
    };
    this.setItem = function(_key, value, overwrite) {
      var key = this.getKey(_key);
      if (this.exists(_key) && !overwrite) {
        return;
      }
      if (typeof value !== "string") return _handleError('"value" must be a string.');
      this.store.setItem(key, value);
      return _key;
    };
    this.getItem = function(_key) {
      var key = this.getKey(_key);
      return this.store.getItem(key);
    };
    this.exists = function(_key) {
      var key = this.getKey(_key);
      return this.store.getItem(key) !== null;
    };
    this.removeItem = function(_key) {
      var key = this.getKey(_key);
      return this.store.removeItem(key);
    };
  }