new ActiveFedora::Auditable mixin - provides access to Fedora audit trail

See: https://github.com/projecthydra/active_fedora/pull/47

unit tests for ActiveFedora::Auditable

Added FOXML fixture for AF::Auditable unit tests

Added Auditable to eager autoload modules

made fixture load work

Requiring Rubydora 1.6; ActiveFedora::Auditable updated