Couchbase::Model#find raises Couchbase::Error::NotFound if id is nil.

fixes #32

Change-Id: Icac4d0eb7e0bb8106d838947a6299dab9b26cd6a
Reviewed-on: http://review.couchbase.org/42053
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>