Add 'retryWhen' operator

[moaible]

Hey guys, I want to add feature that retryWhen operator with retry operator's conditional promise to determine whether retry is possible even if not reached maximum retry count.

Promise<Int> { (resolve, reject, _) in
    currentAttempt += 1
    if currentAttempt < successOnAttempt {
        print("attempt is \(currentAttempt)... reject")
        reject(TestErrors.anotherError)
    } else {
        print("attempt is \(currentAttempt)... resolve")
        resolve(5)
    }
}.retryWhen(retryAttempts) { (remainAttempts, error) -> Promise<Bool> in
    if remainAttempts > retryableRemainAttempt {
        print("retry remainAttempts is \(remainAttempts)... true")
        return Promise<Bool>(resolved: true).defer(5) // I want here promise chain.
    } else {
        print("retry remainAttempts is \(remainAttempts)... false")
        return Promise<Bool>(resolved: false)
    }
}

[malcommac]

Nice addition, thanks!