Release 2.4.0

Added

• update docs/api comments;
• support send cname host req to qingstor;
• ParamType: add enum to replace constants;

Changed

• Multiple refactors and improvements(robust req build procedure);
• Remove several deprecated api that exist from long time ago;
• virtual_host_enabled instead of specify url mode;
• Multiple simplification(local-scope use of stringBuffer, over-encapsulation);

Fixed

• Correct paramType annotation;