0.5.1

CHANGES:

• Updates to address rsync and sandbox issues.
  (@mtelvers #139, reviewed by @tmcgilchrist and @MisterDA)
• Add an obuilder clean command to clean all build results.
  (@MisterDA #140, reviewed by @tmcgilchrist)
• Make rsync-mode mandatory when using rsync store.
  (@tmcgilchrist #132, reviewed by @kit-ty-kate and @MisterDA)