Update DefaultValueValidator.php

Allow the default value to be a closure
Update CHANGELOG.md
Update DefaultValueValidator.php
Check if property is set

```getValue``` didn't check if the property on an object exists so wouldn't return the default value.
limit applied to migration
limit condition changed
Doc comments for the 'mutex' classes extended
Update yii.php
Merge pull request #3624 from makroxyz/master

[skip ci] Italian translation fixes
Mail tutorial created
Mail tutorial finished
Merge branch 'master' of github.com:yiisoft/yii2
fix italian translation
Mail layout charset specification fixed
minor edit [skip ci]
Merge branch 'master' of git://github.com/yiisoft/yii2
CHANGELOG line added
Missing $message var reference removed from mail layouts
Merge branch 'master' of github.com:yiisoft/yii2
Added application component section [skip ci]