Provide append_items/prepend_items

With the Item API, it usually does not make sense to use the '.value'
property of the Item as the value to append. Provided are wrappers that
take a 'fragment' specifier and modify the resulting 'Item.value' field
as appropriate.

Change-Id: Iaa657a3f1257f0943cae56438733d9cb357074d7
Reviewed-on: http://review.couchbase.org/27869
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>