Add ZFS graphdriver:

This work is largely based on the BTRFS graphdriver, and I used
gurjeet's earlier work at
https://github.com/gurjeet/docker/tree/00cb6818/graphdriver/zfs for
reference.

The current implementation shells out to the `zfs` tool. I debated
implementing this with the semi-private `libzfs` and `cgo`, but decided
that:

1) The API of the command-line tool will be more stable; and
2) This was a simpler first step.

If it is desired to implement on `libzfs`, all the actual integration is
squared away in `fs.go`, which should make for a very easy refactor.

Note that this does add a pretty hairy dependency to the dev Dockerfile.
`ubuntu-zfs` takes a while to install, and makes some implicit
assertions about the host kernel, which is fairly unique within the
context of what's there presently. I'm not totally sure how/whether to
work around this.

Note also that I'm not using this in production yet, so trust my code
accordingly.

Signed-off-by: Burke Libbey <burke.libbey@shopify.com>