Revert to real CoreFoundation based plist handling.

This is an implementation of #198.

Instead of using a C extension, this uses the Fiddle API that comes with Ruby >= 1.9.3.

The obvious question is what to do with Ruby 1.8.7. A simple solution would be to keep including the C ext build for a few more versions or alternatively kill 1.8.7 support now.

/cc @CocoaPods/core