update backoff strategy to be exponential

[csantanapr]

The strategy backoff currently is linear with default values of 1second delay with 10 retries.

It should use an exponential backoff instead.
With default values of 10seconds defayl with 10 retries.

Because of OpenWhisk rate limit on trigger limits, it will return 429 statusCode.
OpenWhisk window for the rate limit is 1 minute.

I would make it on first 429 delay for 1 minute, if after 1 minute delay there is a another 429 then I would start exponential back off of 1 seconds (i.e. 2s, 4s, 8s, 16s, 32s, 64s etc..)

[jasonpet]

resolved by PR #90