Diff of NSMutableString

• The added line is THIS COLOR.
• The deleted line is THIS COLOR.
• Go to NSMutableString.

[[NSObject]]

- [[NSString]]

- 文字列の末尾に文字列を追加する（連結・結合）
--  http://www.objectivec-iphone.com/foundation/NSString/stringByAppendingString.html
 NSString => stringByAppendingString:
 NSMutableString => appendString: