ability to manually control overlay's z-index

This would be handy for both user created overlays, as well as those used by angular components such as md-menu.

Take this for example http://plnkr.co/edit/AqjYdsScCYPlK9w44aHQ?p=preview you have a fixed element(some sort custom toolbar), inside it you have a button which triggers md-menu, it works fine, but the `md-menu-click-catcher`, which is for some reason inside the actual `md-menu` element, is actually stacked above `md-overlay-container`, causing any attempt to click on the menu items to close the menu(which can be extremely confusing).
1. `md-overlay-container` should _not_ have z-index at all, as it renders z-indexes of `md-overlay-pane` useless
2. you should be able to manually control z-index of `md-overlay-pane`

I imagine(at least for the user invoked overlay), it could work either by being able to give the overlay my own identificator(class at least), or being able to set it via `OverlayState` settings.. the former is preferable, as you can then easily switch z-indexes with media queries.

まず、overlay の実装と、md-overlay-container、md-overlay-pane、md-menu-click-catcher、OverlayState という名前の要素を特定します。リンクされている md-menu の stacking 問題を再現し、その後、ユーザーが作成した overlay や Angular コンポーネントの overlay を壊さずに、pane の z-index の手動制御と container の z-index の削除をどのように機能させるべきかを判断します。

索引モデルが issue の本文から書いたものです。