Define Couchbase::Result#to_s to produce the same output as #inspect

Change-Id: I9867a479242fecd2cff78516fb999456324d039d
Reviewed-on: http://review.couchbase.org/23701
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Sokolov Yura <funny.falcon@gmail.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>