Added -[NSString toQuantity:]

[mickmaccallum]

Added an additional instance method to the category to return a NSString representing the it's quantity parameter, with the correct pluralization of the receiver appended to it.

e.x.

NSLog(@"%@", [@"thread" toQuantity:0]); // 0 threads
NSLog(@"%@", [@"thread" toQuantity:1]); // 1 thread
NSLog(@"%@", [@"thread" toQuantity:2]); // 2 threads

[mattt]

@0x7fs Thanks for submitting this PR, and apologies for not responding sooner. I think this can be useful for English-only locales, but I'd be concerned about misuse for apps localizing to languages that have other plural forms.