used e.g. for less client side compiling

"Item" parameter keys are the values of the checkbox list,
while the values are the checkbox labels.

code style section has been added here:
https://github.com/yiisoft/yii2/wiki/Core-framework-code-style#self-vs-static

verification code for properties:
https://gist.github.com/cebe/7566916

defaults overided settings

It was mentioned in AssetManager::publish phpdoc.

added version, csrfVar and csrfToken to yii js module.

- Equals sign is now replaced with dot.
- Slash is now replaced with dash.
- Better phpdoc.

- rename FileHelper::mkdir for API consistency
- changed default permission for directories to 775 instead of 777
- added some properties to classes that deal with files to allow control
  over directory permissions.

`generateSalt` parameter validation fix.