Release version 0.5

The most important addition in version 0.5 is the addition of gob-fsck,
a tool that checks an object storage for consistency. Another
improvement for consistency is how gob-chunk handles write errors.
Previously, gob-chunk would just error out and leave the partially
written block in the object storage. Now, it instead tries to unlink it
first and only then abort.

Other than that, there were some refactorings to abstract the object
storage into its own structure. A CI job has been added to do static
analysis on gob's sources via Coverity.