add some default settings

add some default settings
```
+ (void)setDefaultStatus:(NSString*)string; //default is nil
+ (void)setDefaultMaskType:(SVProgressHUDMaskType*)maskType; //default is SVProgressHUDMaskTypeNone
+ (void)setDefaultDismissLeadTime:(NSTimeInterval) second; //default is 0
+ (void)dismissAfter:(NSTimeInterval)duration;
```