Json

Note: not necessarily indending to push this to zfsonlinux.  Creating a pull request seems to be the only way of leaving code review comments?  Let me know if I'm using github wrong.
would it be preferable for the output to have the dataset name as the key?  I think so.  Otherwise, something like "zfs list -J -o used" is basically useless (as it often is without the -J flag, but no reason to repeat that problem here).