Implement bag completeness and validness

[rvanheest]

According to BagIt spec 16 a bag is complete when:

• Every required element is present (bagit.txt, data/, manifest-<alg>.txt)
• Every file listed in every tag manifest is present
• Every file listed in every payload manifest is present
• (BagIt >= 1.0) every payload file is listed in every payload manifest
• Every fetch file is present in the payload

A bag is valid when:

• The bag is complete
• Every checksum in every payload manifest and tag manifest has been successfully verified against the contents of the corresponding file.

Implement methods bag.isComplete and bag.isValid according to the statements above.