Install `rsync` with Homebrew

Use Homebrew to install a modern version of `rsync`. The default install
on older versions of OS X includes options that aren't part of official
rsync distributions. rsync 3 has support for extended attributes.
bin: Fix `rsync` options in backup script

Rely on options that are found in rsync 3.0 (installed via Homebrew)
in order to have the backup script work on a wider range of OS versions.

Use the `rsync` command directly so that the script doesn't have an
explicit reliance on the command mapping to rsync installed in the
`usr/bin` or Homebrew directory.