Handle context cancellations in retry middleware

[csueiras]

Fixes #31

@slok this changeset just handles the context cancellation within the retry middleware as there's no current way for client code to exit out until the retry attempts are met. I have a use case where I basically want to retry until:

• Context is cancelled (thus this changeset)
• A non-retriable error occurs

If the context is cancelled pre-execution of the retry middleware the middleware will just continuously retry until the attempts are met (in my use case I basically want to retry forever (math.MaxInt). If the context gets cancelled through a deadline the retrier is unaware of the context cancellation so again it will continue to retry.

[muzavan]

If the feature ended up merged, the behavior upon context.Canceled should be specifically mentioned in the comment (probably on New, or maybe using a flag as part of config).

What do you think?