Disable audit-mode by default.

* Modify command line option --audit-mode to accept parameters enabled, disabled, or audit-only.
* Emit a warning if audit-mode is enabled or audit-only.
Merge branch 'audit-mode' of github.com:opscode/chef into mcquin/audit-mode-experimental
Add specs for audit-mode command-line and configuration settings.
Clean typos.
Explicitly enable audits.
Stub Chef::Log#warn.
Merge pull request #2622 from opscode/mcquin/audit-mode-experimental

Disable audit-mode by default.