Rewrite plist saving to use CoreFoundation API

[kylef]

There are many issues with the implementation introduced in 0.7 causing strange crashes when using CFPropertyList, along with other problems.

@alloy has found a solution and is going to rewrite the implementation using fiddle which allows us to load CoreFoundation and call methods on it without making a C extension. Fiddle is included in Ruby 1.9.3 so it will be pre-installed on most ruby. For those who don't have it (Ruby 1.8.7) it would result in compiling the Fiddle C extension.