Fixes #706: Added `yii\widgets\Pjax` and enhanced `GridView` to work with `Pjax` to support AJAX-update

which resulted in endless loop

http://www.yiiframework.com/forum/index.php/topic/50982-cfilehelper-bug-security-flaw/

Added `yii\web\Request::get($name = null, $defaultValue = null)` and `yii\web\Request::post($name = null, $defaultValue = null)`

Fixes #2157: The '*' category pattern will match all categories that do not match any other patterns listed in `I18N::translations`

fixes #2132, fixes #2133

Added one line about support of UNION ALL queries.

- i18n now falls back to `en` from `en-US` if message translation isn't found
- View now falls back to `en` from `en-US` if file not found
- Default `sourceLanguage` and `language` are now `en`

Fixes #1958: `beforeSubmit` in `yii.activeform` is now executed after validation and before form submission

fixes #1585

issue #2043

added a fallback for handling all request types

fixes #2066, related to #2043

Fixes #2063: Removed `yii\web\Request::acceptTypes` and renamed `yii\web\Request::acceptedContentTypes` to `acceptableContentTypes`.
Renamed `yii\web\Request::acceptedLanguages` to `acceptableLanguages`