Serbian translation. Fixes #1907
Fixes #1909
Fixes #1900.
Fixes #1809: Added support for building "EXISTS" and "NOT EXISTS" query conditions
remove sr-cyrl, it's the same as sr
rearanged language array
Merge branch '1907-sr-language' of https://github.com/johonunu/yii2 into johonunu-1907-sr-language

* '1907-sr-language' of https://github.com/johonunu/yii2:
  rearanged language array
  remove sr-cyrl, it's the same as sr
  Serbian translation. Fixes #1907

Conflicts:
	framework/messages/config.php
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.
Renamed Request::maskedCsrfToken to csrfToken.
Gii excess forms. Fixes #1818
changed vars
using camelcase