Add --dry-run option to prune.
Merge pull request #43 from jdchristensen/prune-dry-run

Add --dry-run option to prune.
Automatically replay segments to rebuild missing repository index
repository: Fix potential race condition

If we crash between compact_segments() and write_index() and the
transaction deletes objects that are newer than the current index
might become undeleted.
Ignore xattr errors during extract if not supported by the filesystem

Closes #46.
repository: Fixed a potential infinite loop in recover_segment
check: archive metadata recovery improvements
check: More code cleanup