Update version to 0.21-SNAPSHOT.
Add pom elements required by Sonatype
Update CoreJet version to 0.21 in preparation for release
Add logging for stories with no estimate.
Enable greater configurability of offline mode.

Changing the default behaviour of JiraStoryRepository to delegate to OfflineJiraStoryRepository if a recent cached requirements file exists.  If the cached requirements file is more than one hour old (or does not exist), delegate to OnlineJiraStoryRepository.

Ensuring that system properties can be set to force working offline or online.