New options: --data-source, --quiet, and --rsh

Quite the flurry of new features today, although they're somewhat niche.

For anyone who wants to do consistent "rolling" snapshots, the
--data-source will be useful, by letting you specify any other block
device as the one to read data blocks from. Specifying completely the wrong
block device will end in tears, so don't make a mistake...

If knowing how much time you've saved annoys you, the --quiet option will
cause lvmsync to only display fatal error messages.

Finally, if you want to run something other than just ssh as your network
transport, the --rsh option will suit you perfectly.