Retry with backoff

A retry function (with backoff) would be neat. Something like - 

```
retry(request: Promise<Request>, retryCap: Int = 5, retryInterval: TimeInterval = 0.5)
```

Or some other syntax if anyone else has any better ideas. 

I'd be happy to implement this; @vadymmarkov is this within what you'd like Malibu to be?