Added NSArray -sortWithBlock, -uniq, -indexOf and -partition

I just switched my project to use ObjectiveSugar and I want these too. I also added [NSArray compare:], which is pretty much required in order for sortWithBlock to work rationally. It's easy to implement and just a useful thing to have in general. Hopefully I did everything correctly. :)

If you like these and accept the pull request, I have more things I want to add...

One suggestion - would you consider revamping or getting rid of the examples in the README? It feels hard to maintain and error prone. Maybe something like this instead?

https://github.com/zdavison/NSString-Ruby
(see the Method List section)