Add simple Retry implementation

[joeycumines]

I enjoy how simple Backoff is, but I found it surprisingly difficult to use it to correctly implement retry functionality.

This PR contains a single, very simple Retry implementation, that I've tried to style in the same vein as the existing code.

It is fully tested with examples, please take a look.

[joeycumines]

@jpillora I did see you closed #12 for being too high level.

I feel like this approach provides excellent utility, still allowing arbitrary retry behavior. That is just IMO though, I am curious to see what you think.

Thanks for this implementation btw, I've used it many times now.