Changed usage of self to static to allow extendibility

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
Merge pull request #1258 from cebe/self-static

Changed usage of self to static to allow extendibility