Lumberjack key improvements, agent tweaks

- Add some notes about ::agent usage
- Add fall through logic to try an encrypted data bag, then regular data bag, then auto-generate keypair
- Add unit test for the new logic where you don't have to supply a keypair
Fix test logic by setting attribute in chefspec, add one more conditional to agent recipe
Merge pull request #51 from martinb3/master

Lumberjack key improvements, agent tweaks