Default finder finds by id. Closes fauna/issues#20.
URI.escape refs at the model level. Fixes problems with route-based finders.
Allow a publisher key to be specified in fauna.yml

Now a publisher key can be specified directly in fauna.yml,
allowing the email and password credentials to be omitted and
avoiding the creating of a new publisher key every time the
app is booted.
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 fauna.yml
credentials.