[PlistHelper] First version of old C ext ported to use Fiddle.

This means we link against the real CoreFoundation framework, but there
is no need for C ext compilation.
[PlistHelper] Replace deprecated CoreFoundation APIs.
[PlistHelper] Use actual signatures for all CF functions.
[PlistHelper] Refactor importing CF functions.
[PlistHelper] Cleanup function definitions.
[PlistHelper] Re-organize and clean-up more.
[PlistHelper] Re-organize a bit more to reduce namespace noise.
[PlistHelper] Merge and clean-up tests.
[PlistHelper] Make tests green and remove XML fixture.
[PlistHelper] Make rubocop happy.
Remove any further reference to the CFPropertyList and libxml-ruby gems.
[PlistHelper] Import build of old C ext for Ruby 1.8.7 only.
[PlistHelper] Actually use old API that's used in C ext.
[PlistHelper] Don't change name of C ext bundle as Ruby inflects the init function from the filename.
[PlistHelper] Don't reference constant at all to ensure the file is not autoloader on Ruby 1.8.7
[PlistHelper] Remove more references of new constant.
[PlistHelper] Remove unused code.
[PlistHelper] Some more cleanup.
[CHANGELOG] Use Fiddle API to use CoreFoundation's CFPropertyList APIs.
[CHANGELOG] Formatting fix.