add option to release public ip on machine destroy

Author: sanfilip

lib/machines/destroy.js

@@ -14,6 +14,7 @@ var assign = require('lodash.assign');
  * a team, the team administrator.
  * @param {object} params - Machine destroy parameters
  * @param {string} params.machineId - The id of the machine to destroy
+ * @param {boolean} [params.releasePublicIp] - releases any assigned public ip address for the machine; defaults to false
  * @param {function} cb - Node-style error-first callback function
  * @example
  * paperspace.machines.destroy({