Adding Cancel and CancelToken classes

lib/cancel/Cancel.js

@@ -0,0 +1,17 @@
+'use strict';
+
+/**
+ * A `Cancel` is an object that is thrown when an operation is canceled.
+ *
+ * @class
+ * @param {string=} message The message.
+ */
+function Cancel(message) {
+  this.message = message;
+}
+
+Cancel.prototype.toString = function toString() {
+  return 'Cancel' + (this.message ? ': ' + this.message : '');
+};
+
+module.exports = Cancel;