[objective-c] add Anagram exercise
[objective-c] initial Anagram implementation

http://exercism.io/submissions/b87cbd8676cbeac3177e5ce9

I wanted to experiment with adding a category method to NSString for
this exercise instead of keeping it all in the `Anagram` class.
[objective-c] cleanup Anagram sorts
[objective-c] refactor Anagram to use block

@andrewsardone helped me refactor this to use a block, mostly to give me
some practice with it.  Unsure if this is a "better" solution than
adding it as a category, but it is kind of cool.