[iOS] return UDID as NSString

iOS UDID

+ (NSString *)UDIDString {
	NSString *udid = (NSString *)CFURLCreateStringByAddingPercentEscapes(NULL,
                                                                          (CFStringRef)[[UIDevice currentDevice] uniqueIdentifier],
                                                                          NULL,
                                                                          (CFStringRef)@" !*'();:@&=+$,/?%#[]",
                                                                          kCFStringEncodingUTF8);
	return [udid autorelease];
}