Add templates for Mac OS X 10.7, 10.8, 10.9.

@juliandunn @sethvargo @schisamo @fnichol

Add packer templates for Mac OS X 10.7, 10.8, 10.9.  These are taken more or less wholesale from @fnichol's https://github.com/fnichol/packer-templates and are attributed as such in the README.