Getting selected html content in tinymce editor

I have created a custom button using this code

    setup : function(ed) {
    ed.addButton('Tittle', {
                title : 'Tittle',
                image : './images/T.jpg',
                onclick : function() {
                ed.focus();
            var c = ed.selection.getNode().nodeName;
        if(c!="TITTLE")
        {
             ed.selection.setContent('<tittle>' + ed.selection.getContent() + '</tittle>');

        }
        else
        {

        }
}
        });

When a user select a text and click on the new button, i want to add a <title> tag at beginning and ending, if <tittle> tag is not their.If <tittle> tag is already their in the selected text i want to remove the tag