Please add promisified API for use with async/await in upcoming Node 7

[kristianmandrup]

F.ex use https://www.npmjs.com/package/promisify-node

[AckerApple]

My pull request adds a method of require('node-dir').promiseFiles(dir, options).

Check it out, working with package owner to make official: #40

fork location: https://github.com/AckerApple/node-dir

[mandricore]

Good stuff :)