Add Ability To Destroy From jQuery Command

Adds the ability to call the destroy method directly from the jQuery
function

This change adds the ability to call the destroy method from the jQuery
function.  You call it by passing in a destroy property set to true into
the dragend method:

$('#dragendcont').dragend( {destroy: true} );

Without this change you needed to have a instance of the dragend object
to call the destory method:

var instance = new Dragend();
instance.destroy();