PYCBC-157: Provide FMT_AUTO

This pseudo-flag has the client "guess" the appropriate intended
encoding of the object.

Change-Id: I38487fb114c5580b90d59c5068da8e584b89c2b3
Reviewed-on: http://review.couchbase.org/27652
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>