Naming files, classes and functions

[rianrietveld]

The following names cause errors with phpcs

File naming

• Class file names should be based on the class name with "class-" prepended.
• Filenames should be all lowercase with hyphens as word separators. This is the case for the files in the wordpress-objects dir.
• Method name "_refresh_data" should not be prefixed with an underscore to indicate visibility

I propose to rename those files/methods.

This also means the we need to move the class Clarkson_Coreoutside the main clarkson-core.php file.