WPCS compliance

[adamziel]

Using php-toolkit in WordPress core and core plugins (such as wordpress-importer) requires compliance with WordPress coding standards. Let's make this codebase compliant.

Related work:

• Add SlevomatCodingStandard.ControlStructures.DisallowYodaComparison sniff #156
• Update PHPCS versions to stable ones #155
• Rename class files to comply with WPCS #153

cc @JanJakes