- Require credentials are explicitly created
- Emit signal when craeting new credentials for hooks
- Docstrings for more cred classes
- Do not default revision to null, use 'queue' to denote
  unsent objects instead. null is not valid for an indexed
  property.
Merge branch 'master' into 29-transactions
[#29] In all I/O operations, use the new transaction wrapper. Plasmid now handles multi-operation transactions.