Merge branch 'allow-publisher-key'

* allow-publisher-key:
  Allow a publisher key to be specified in fauna.yml
Merge branch 'allow-publisher-key'

* allow-publisher-key:
  Print the publisher key to STDERR during boot if present.
Rails helper falls back to email/password if publisher_key is nil.

Docs updated to explain the order of precedence in fauana.yml
credentials.