JCLOUDS-457: Add polling strategy

The polling strategy interface for job operations
and a simple implementation class have been added.
This implementation waits an initial time and then
polls at regular intervals.
It seemed correct to me allowing interruption. Also, if the user works with the provider instead of the abstraction and uses the PollingStrategy, they can handle the interruption as they please.

Any thoughts about this @andrewgaul ?