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.
Move main plugin class into its own file.
Define contants for the URL and path to the plugin's main directory.
Update paths and URLs to use the new constants.
Move the  property to the toolbar class.
Add dot org readme and update version number.
Add a check to regenerate the plugins transient if upgrading from 1.0 to 1.1