Pass -H and -S by default to rsync. Mention -A and -X.

Idea from Lasse Collin.
Remove --delete and --delete-after switches.
Remove -o PasswordAuthentication=no because -o BatchMode=yes
already prevents password querying.

Idea from Lasse Collin.
Use invalid@invalid.invalid rather than foo@bar.com for
ssh command.

    https://tools.ietf.org/html/rfc6761
    https://tools.ietf.org/html/rfc2606

Idea from Lasse Collin.
Don't clean snapshots if user has passed -n switch for dry run.

Idea from Lasse Collin.
Make sure that locks/ exists.