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 input-validation.md
Update CHANGELOG.md
Update CHANGELOG.md
Update yii.activeForm.js

Refactored
Update input-validation.md

Updated guide
add page start-forms.md
Grammar fix
add start-looking-ahead.md
Using of `MongoCollection::createIndex()` provided
Merge pull request #4539 from mickgeek/master

Fix signature validation
- Enh #4520: yii\caching/MemCache.php add sasl support (xjflyttp)
Merge pull request #4542 from xjflyttp/4520-cache-memcache-sasl-support

- Enh #4520: yii\caching/MemCache.php add sasl support (xjflyttp)
minor fixes [skip ci]
Refactored ActiveQuery::one() to be consistent with all() creation
Added tests that are making sure Gii isn't failing with errors
Merge pull request #4544 from yiisoft/gii-generator-tests

Added tests that are making sure Gii isn't failing with errors