Added --list option to zbackup.

This summarises for all the ZFS filesystems on the system what
backup properties are set, and therefore what zbackup will do.
Updated README for --list option of zbackup.
Fixed blocking bug in e74563f (--list option), oops.
Added zbackup options --set and --unset, for managing backup properties.
Broke out README for zbackup into a separate file, and expanded documentation.
Improved zbackup error reporting; added capability to email on failure.
Merge pull request #10 from tesujimath/master

zbackup enhancements: set/unset/list; error reporting; restructured documentation