fixed build break and fixed doc about exists/not exists.
renamed sr-latn to sr-Latn.
code style fix
added doc about translating message.
Merge branch 'master' of github.com:yiisoft/yii2
Added futter. Fixes #1822
removed failing tests.
Merge pull request #1912 from johonunu/1822-sticky-futter

Added sticky futter. Fixes #1822
fixed test break.