Fixed YiiBaseTest echo

it echoed Yii version which is not necessary
created StringHelper::basename()

In contrast to php function basename() It that always uses \ and /
as directory separator.
It is needed for use on Classnames that contain namespaces.
Fixes #192
Renamed yii.php -> Yii.php for class, filename consitency

See https://github.com/yiisoft/yii2/issues/123#issuecomment-17657374
Issue #123