Some more general fixes for the Item API

- Provide clear warnings about not overriding 'Item' fields as
  desciptors
- Provide test to ensure it doesn't blow up (i.e. crash or segfault) if
  we do that.
- Provide handy constructor of Item(key, value)

Change-Id: I811f5b7f508d22ca688fb1bdb1070a51178ddeee
Reviewed-on: http://review.couchbase.org/27919
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>