feat(menu): add menu trigger support

[kara]

This PR adds basic menu toggling functionality (see design doc here).

For the below, clicking the button will automatically toggle the menu.

<button [md-menu-trigger-for]="menu">More</button>
<md-menu #menu="mdMenu">
   <button md-menu-item>Item one</button>
</md-menu>

Still TODO (in future PRs):

• Animation for menu opening
• Position setting
• Keyboard events
• Prevent-close attribute

[jelbourn]

Just realized we could do this as String(this.disabled), which is slightly shorter.

[aminebizid]

Menu shoould be closed when clicking outside or resizing the window (body:click & window:resize)
A z-index strategy should be managed in overlay class (adding a menu on a dialog, nested menus, etc.)

[jelbourn]

I just realized that it might be a problem in Angular Universal if we try to create the overlay on init rather than once a user interaction occurs, since under the hood the overlay creates an element with browser APIs.

[kara]

Hmm, good point. I'll check it out.

[jelbourn]

LGTM

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