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.
`describeJob` is [not documented](https://github.com/jclouds/jclouds-labs-aws/blob/master/glacier/src/main/java/org/jclouds/glacier/GlacierClient.java#L248) as returning `null`. Can it? If so, update the Javadoc to explain that?