Add start of Mac support, DRY up downloads

- Mac packages come from downloads.puppetlabs.com
- Use interpolation for $filename instead
  of repeating it in each variation
- Fix a gap in logic: if both a commandline name
  and a download directory were specified, previously
  the name would be ignored. Now it's two separate
  conditionals.
Fix writing stderr files for error checking

The routines which seached through the stderr
of the download commands for problems used the
`$tmp_stderr` variable, but the download programs
themselves wrote to /tmp/stderr, so the errors
were not found.
More complete Mac OS X support

This adds installation for Macs via dmg, which
are actually a pkg-inside-dmg and therefore require
mounting, then installation.

There's more to be done; specifically this -only-
installs Puppet and therefore does not leave you
with a working setup -- facter and hiera are
separate downloads and there's no automatic dependency
resolution in Apple-land.
Installer requires root access apparently
* Added in info pane to warn it might ask for sudo
* Added sudo to the installer command