Set a callback for when caching is activated for a path.

@param {function} callback - Callback function
function (cb) {
    var i;
    log('[Caching] Setting activate handler', cb, this.pendingActivations);
    this.activateHandler = cb;
    for (i=0; i<this.pendingActivations.length; i++) {
      cb(this.pendingActivations[i]);
    }
    delete this.pendingActivations;
  }