LLVM 4.0 新特性，再也不用恶心的 @synthesize 了

Xcode now includes the Apple LLVM Compiler version 4.0, including the following newObjective-C language features:

 - Default @synthesize: automatically synthesizes an @property when unimplemented 
 - Objective-C literals: create literals for NSArray, NSDictionary, and NSNumber, just the same as the literals for NSString 
 - Objective-C container subscripting: use '[]' syntax to access containers