FMT_AUTO fixups:

Add some more tests for FMT_AUTO
Allow FMT_AUTO to be specify as default_format
Make FMT_AUTO an instance of a more intuitively named subclass to show
more understandable errors during attempted integer operations

Change-Id: Ie525781b738fa9c9ca915eb6905d4482a2b02d41
Reviewed-on: http://review.couchbase.org/28231
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>