RBAC: Item and Assignment optimized save approach

Renamed yiirequirements.php to requirements.php

"YiiRequirementChecker" console view has been reworked to display requirements by blocks and in brief for the successful ones.

Yii core requirements file has been composed.

issue #237

https://github.com/yiisoft/yii2/commit/e2513de6c6207fa37894a944e10b8cd420ac6804#commitcomment-3192244

"YiiRequirementChecker::check()" has been updated to be able to collect different requirements set in the chained call.

as discussed in #203
https://github.com/yiisoft/yii2/issues/203#issuecomment-17759631

Methods "YiiRequirementChecker::getResults()" and "YiiRequirementChecker::render()" have been added.

RBAC: DbManager code style

change is to avoid random test failure on race condition.
fixes #203 #231