Updated nokogiri-plist -> plist.

nokogiri-plist doesn't appear to want to play nicely under ruby 1.9.3. Luckily, the api is almost identical to the more well known plist gem: https://rubygems.org/gems/plist

I also converted all of the tabs to the more accepted ruby style of two spaces per indentation level. Well, my editor did, anyway.

Also added a readme.