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

Given the time these operations can take, that makes sense to me too. But from what I recall we catch `InterruptedException` in GlacierBlobstore and rethrow as a RuntimeException, so we don't really give you that much "nice" control as a user.

In any case, would it make sense to document (probably on GlacierBlobstore) that it is "safe" to interrupt the long-running operations, and giving a code example (in the Javadocs or jclouds-examples) showing how to do that..?