add_action( 'init', array( $this, 'btn_mapping' ) );
add_action('admin_enqueue_scripts', array( $this, 'btn_scripts' ) );

---------------------------------

public function btn_mapping(){

	//Abort early if the user will never see TinyMCE
	if ( ! current_user_can('edit_posts') && ! current_user_can('edit_pages') && get_user_option('rich_editing') == 'true')
		return;

	//Add a callback to regiser our tinymce plugin
	add_filter("mce_external_plugins", array( $this, "btn_register" ) );

	// Add a callback to add our button to the TinyMCE toolbar
	add_filter('mce_buttons', array( $this, 'btn_add' ) );

}

public function btn_register($plugin_array){
	$plugin_array['exeter_btn'] = EXETER_COMPANION_URI . '/assets/js/editor-button.js';
	return $plugin_array;
}

public function btn_add($buttons){
	$buttons[] = "exeter_btn";
	return $buttons;
}

public function btn_scripts(){
	wp_enqueue_style('exeter-tc', EXETER_COMPANION_URI . '/assets/css/editor.css');
}