Signed-off-by: qiansen1386 <qiansen1386@gmail.com>

thanks to @chris68 PR #1338

Removed error-block HTML class and related styles and used Bootstrap3 standard practice about errors instead

fixes #1821

Fixes #1835: `CheckboxColumn` now renders checkboxes whose values are the corresponding data key values

Changed Yii autoloader to support loading PSR-4 classes only (i.e. PEAR-styled classes not supported anymore)

$cost must be in range 4-31.