removed specify dependency from extension to app

fixes #1913
fixes #1888: updated Alert phpdoc
Added CONTRIBUTING linking to git workflow doc
Actual text for contribution guideline, renamed file to use .md
fixes #1674: Updated contributing docs
Added wiki articles as docs
Update core-code-style.md
Re-added code style :)
Updated Russian translation not to repeat values for plurals
Updated changelog
Fixes #1870: Validation errors weren't properly translated when using clientside validation
Fixes #1905: Update composer.json to use jQuery 2.0.*
Fixed yii2-dev package to use correct path for PSR-4
Used dirname instead of realpath to avoid extra FS operations
Added note about configuring theme do docs
Update yii.php

Fixed missing translation
Merge pull request #1936 from Theill11/patch-1

Fixed missing danish translation
fixes #1870: better fix that is message extractor friendly
Removed debug code
phpdoc fixes, unused variable