Switch the platform statement to platform_family

darwin -> mac_os_x
Change tabs to spaces to fix formatting
Add oracle and mac_os_x_server as supported platforms
Add chef/ohai requirement to the readme
Change OS X attribute selection to platform_family as well

This allows this recipe to work on mac_os_x_server platforms
Merge pull request #13 from tas50/COOK-2272

Cook 2272: Use platform_family throughout to add Oracle Linux and Mac OS X Server support