Implement OAuth authentication result deleting

Add functionality when the result of OAuth access token auth can
be deleted directly, without the need to fetch and construct a new
resource. Shave off several API calls.

lib/oauth/stormpath-access-token-authentication-result.js

@@ -99,4 +99,17 @@ StormpathAccessTokenAuthenticationResult.prototype.getApplication = function get
   this.client.getApplication(this.application.href, args.options, require('../resource/Application'), args.callback);
 };
 
+/**
+ * @function
+ *
+ * @description Delete the access token used to authenticate.
+ *
+ * @param {Function} callback
+ * The callback to call after the resource is deleted, with parameters (err, null)
+ */
+StormpathAccessTokenAuthenticationResult.prototype.deleteAccessToken = function deleteAccessToken(/* callback */) {
+  var args = utils.resolveArgs(arguments, ['callback']);
+  this.client.deleteResource({href: this.href}, args.callback);
+};
+
 module.exports = StormpathAccessTokenAuthenticationResult;