Reorganise code

[GaryJones]

• Move main plugin class to its own file.
• Move other non-view PHP files into /src directory.
• Rename classes to be prefixed with class- per WPCS (though this may change to use PSR-4 naming in the future once namespacing and autoloading is considered).
• Simplify CSS and JS file locations.
• Simplify defined constants in the root file.
• Update plugin headers to be more detailed.
• Add namespace to root file.
• Change how the main class is initialised.