deferred/promise

I'm not really convinced whether it's needed in rimraf, but when we compare solutions I think promises should be there as well.

It's plain EcmaScript but to work needs [deferred @0.2](https://github.com/medikoo/deferred) and [es5-ext @0.6](https://github.com/medikoo/es5-ext) packages installed (I haven't added them to project dependencies). Solution uses deferred package, with some extra functional sugar from es5-ext.

I'll be happy to know our opinion on that.

Just to clear one thing, I think deferred solution is right when we deal with many asynchronous calls that need to be joined, chained, nested, whatever - rimraf is quite good example. However I wouldn't use promises for writing one basic asynchronous function, and it's great that Node doesn't force us to use promises for nearly anything asynchronous (as it was for a while).