FileHelper port from 1.1.x

pgsql schema findColumns fix

API consistency in Response class

use `getIs...` instead of `is...` to allow these methods to be used as
virtual properties.

This is forconsistency to the rest of the framework.
Only methods that take parameters may be named `is...`.

Doc comments at "yii\helpers\base\FileHelper::findFiles()" have been recomposed using markdown format.

Migrate command primary key

Issue 466 (add/drop PrimaryKeys)