added NSSortDescriptor option sort time

NSSortDescriptor is much faster than any other sort method available in NSArray. Here is the results on my computer .  

2014-01-08 23:20:31.112 PSTFoundationBenchmark[36326:907] Sorting 1000000 elements. selector: 6926.93[ms] function: 9356.48[ms] block: 5662.77[ms] sortDescriptor: 2650.73[ms].