Fix SNAPNAME to contain the DATE if label is empty

When I didn't specify a `--label` on the commandline, the SNAPNAME was missing the $DATE component
Merge pull request #25 from stuehmer/patch-1

Fix SNAPNAME to contain the DATE if label is empty
Makefile:
  * Remove PREFIX from cron scripts - they should always end up in /etc.
  * Leave PREFIX for man page and program, and set it to /usr/local by default.
Add `--{pre,post}-snapshot` for pre and post creation hook
Abort if pre snapshot hook returns non-zero
Document `--{pre,post}-snapshot
Make --{pre,post}-snapshot optional, add --destroy-only, revise docs
Update README

Add some simple installation instructions
Merge pull request #26 from borutmrak/master

Fix manpage path
Merge pull request #29 from mmalecki/pre-post

Add pre and post snapshot hooks
Merge pull request #30 from mbaynton/remove-only

--destroy-only switch, --{pre,post} fixes
Merge pull request #31 from virtualguy/patch-1

Update README
Install the cron.d part without the execute bit.

Resolve this lintian error:

	E: zfs-auto-snapshot: bad-permissions-for-etc-cron.d-script etc/cron.d/zfs-auto-snapshot 0755 != 0644
	W: zfs-auto-snapshot: executable-not-elf-or-script etc/cron.d/zfs-auto-snapshot

This should also resolve a related failure on Arch Linux.

Closes: zfsonlinux/zfs-auto-snapshot#4
Thanks: @bitloggerig
Thanks: @scottj97
Merge branch 'master' into debian
Update debian/rules for Makefile changes.

Create an override_dh_auto_install rule that appropriately forces the PREFIX
for a managed installation. Creating autotools boilerplate or a fancier
Makefile for this script is not yet worthwhile.
Update changelog for 1.2.1-1 release