Updating readme to have example of UIBarButtonItem

I'm a complete idiot and spent WAYYYY too long messing with UIBarButtonItem before I realized it took a block parameter.  
Updating README to save someone else the time.
add NSString#cache
Easy to get NSSet from Array and also reverse.
fix with enumerable methods.
NSURL -> NSData
Merge pull request #56 from matsu911/master

implemented NSURL -> NSData conversion method
Merge pull request #54 from katsuyoshi/nested_anonymous

fix with enumerable methods.
Merge pull request #53 from katsuyoshi/array_set

Easy to get NSSet from Array and also reverse.
Merge pull request #52 from katsuyoshi/cache_dir

add NSString#cache
Merge pull request #51 from GantMan/master

Adding Example to README