STORM-495: KafkaSpout retries with exponential backoff

This is an implementation of the improvement request STORM-495:
    https://issues.apache.org/jira/browse/STORM-495

The KafkaSpout keeps track of failed messages through multiple retries, and delays their resubmission to the topology with an exponentially increasing delay time.  The spout does not terminate the retry cycle (it is my conviction that it should not do so, because it cannot report context about the failure that happened to abort the reqeust), it only handles delaying the retries.  A bolt in the topology is still expected to eventually call ack() instead of fail() to stop the cycle.
Thanks for the clarification @rick-kilgore. If we can do something loud and obnoxious (I.e. Logging) in the fail method to let users know what is going on, that would help. I'd like to avoid allowing users potentially shooting themselves in the foot and getting into a difficult to debug situation.

I'd also like to see some documentation in the README in that respect. 

Thanks again!