Add extension support to yii\twig\ViewRenderer
a note about how to not use gii

related to #1482
Merge pull request #1487 from nergal/twig-viewrenderer

Add extensions support to yii\twig\ViewRenderer
gii docs
updated gii docs
use andWhere() in AR::find() to work properly with default scope

fixes #1469
More docs
Fixed `Call to a member function registerAssetFiles() on a non-object` in case of wrong `sourcePath` for an asset bundle
Added docs about using yii2-dev package
Linked to yii2-dev docs from README
Adjusted exception template. Background of images now matches background color of the header.
removed composer.lock from gitignore

we should not provide one in the package but developers should commit
them when starting a new project so that all share the same versions
and only explicit composer update command will change that.
Escpecially important when deploying to production.
Merge pull request #1492 from egorpromo/more-docs-for-event

More docs for /yii/base/Component.php::trigger()
doc style
Added favicons to application templates
Fixes #1497 (wrong file rendered if language != sourceLanguage)
Merge pull request #1498 from mintao/master

i18N: View path does not change if language folder exists
updated change log.
Added robots.txt to application templates
Updated changelog: added line about app template addition of favicon.ico and robots.txt