PYCBC-156: Item API

Item represents a user-subclassable, in/out object which can be passed
instead of an explicit "key-value" pair for most of our APIs.

Change-Id: Iadb54653c464b32bf0373092f0b0a8c0edbcbb94
Reviewed-on: http://review.couchbase.org/27620
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>