Changed elasticsearch AR primary key handling

now supports mapped primary key when _id is part of source with an
alias.
added german translation

thanks to @chris68 PR #1338
Added link to pull requesting guide
updated autoloader phpdoc
Merge branch 'master' of github.com:yiisoft/yii2

* 'master' of github.com:yiisoft/yii2:
  Added link to pull requesting guide
Add Italian Language
typo. fixes #1858
Better exception message when class cannot be loaded
Fix table name on yii\rbac\DbManager::$itemChildTable
Merge pull request #1860 from LAV45/master

Fix table name in yii\rbac\DbManager::$itemChildTable
Merge pull request #1861 from gimox/gimox

Add Italian Language
reverted primaryKey of elasticsearch to be array again

meet the AR interface requirements.
Updates pt-BR translation
Merge pull request #1862 from andersonamuller/patch-1

Updates pt-BR translation
improved error reporting
Update yii.php
Removed exception translation, updated translations
Italian Translation Typo Fix
Changed Mongo → MongoDB in exception messages and logs
Update Language Translation