Fix removal of `".vex-open"` body class when `animationEndSupport` is false

When `animationEndSupport` is false, `vex.getAllVexes().length` was `1` in the check to remove the `vex-open` body class name. By introducing a `vexAfterClose` event fired on the body, we guarantee (for both `animationEndSupport` and not) that our check for any remaining vexes on the page will come _after_ the `$vex` element removal has actually happened. (Fixes #46)