Refactoring to make code more self-documenting.

Extract methods, change visibility scope of non-hook callback methods etc.
Removed the redundant function exists check for 'get_plugins' for now.
Wrap comments at 80 characters.
Refactor toolbar and plugin objects into separate classes/files.
Move CSS and JS files into an assets directory to keep the main folder organized.
Update the readme.
Move main plugin class into its own file.