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.

首先定位 overlay 的实现以及名为 md-overlay-container、md-overlay-pane、md-menu-click-catcher 和 OverlayState 的元素。复现所链接的 md-menu stacking 问题，然后确定如何实现对 pane 的 z-index 进行手动控制以及移除 container 的 z-index，同时不破坏用户创建的 overlay 或 Angular 组件 overlay。

由索引模型根据 Issue 内容生成。