add guard-rsync and two Guardfiles for two different approaches to merging

The documentation is currently in the comments header of each Guardfile - still need to add something to the devguide.

It's not the most elegant solution due to the DRY violation, but it should be useful in a lot of cases.

See http://publishwith.me/crowbar-tree-merging for the design background.

Currently uses `guard-rsync` from my github repo; if/when kselden/guard-rsync#8 is accepted, this can be changed to point back to the upstream version.

Please don't merge this until at least one person other than myself has tested and +1'd !