'how to use' first
refine return type signature
nil safety
add NSASubscript, update NSADataObject
add/fix array/dictionary extensions

rename NSArray popRandomObject to removeRandomObject
add anyKey, anyObject to NSDictioanry
add shuffledArray to NSArray
fix NSArray randomObject, randomObjectsOfCount: behavior similar to anyObject
NSCountedSet functional support
mark 0.51