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.
Is there any specific reason to allow interruption here (given the long potential runtime of Glacier, perhaps yes)? Otherwise, consider [`sleepUninterruptibly`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/util/concurrent/Uninterruptibles.html#sleepUninterruptibly%28long,%20java.util.concurrent.TimeUnit%29)?