deferred validation

Added support of deferred to ```ActiveForm``` validation.
deferred validation

Added support of deferred validation.
Update yii.activeForm.js

remove empty message arrays
Update yii.activeForm.js
Update CHANGELOG.md
Update CHANGELOG.md
Update yii.activeForm.js

Refactored
Merge branch 'deferred-validation' of github.com:Alex-Code/yii2 into Alex-Code-deferred-validation

Conflicts:
	framework/CHANGELOG.md