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

"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.

app is resposible for setting time zone, so it should also do the
fixing.
+ documentation about the behavior.
issue #210 and #209

set application timezone early in application constructor, set default of UTC if no timezone is configured at all (not in app config nor in php.ini)