Add retry support using node-backoff.

@rcoup - I like this patch, though it seems like it needs to distinguish between 4xx responses and internal/upstream errors in order to not repeatedly hammer URLs that will never work.  How's it been working for you?

https://github.com/mapbox/tilelive.js/pull/66#issuecomment-40444919 should help address that, as the 404 condition won't pass an error forward.

Thoughts?