PYCBC-196: Convenience methods for Item API

This adds some additional convenience methods and documentation for the
Item API.

Change-Id: I70835ab514f4097d737d52719bf7bab1ec6baad4
Reviewed-on: http://review.couchbase.org/29304
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>