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.

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

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