api3: add LCB_VALUE_SIMPLE

Change-Id: Id60cf5fa6ba69fca1aed363e3861f8ee03cccf53
Reviewed-on: http://review.couchbase.org/38845
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>