feat(peekaboo): decorator for viewport scroll actions

 - mimic the component used on the google design page.
 - apply an action when the peekaboo breakpoint is triggered. support 'show' and 'hide' actions.
 - TODO: mapping of viewport sizes (e.g. 'sm','lg','md') to peekaboo breakpoints. Use to support different breakpoints on smaller viewports.
 - TODO: tests

ng2-material/components/peekaboo/peekaboo.scss

@@ -0,0 +1,14 @@
+[md-peekaboo] {
+  &[action=show] {
+    display: none;
+    &.md-peekaboo-active {
+      display: inherit;
+    }
+  }
+  &[action=hide] {
+    display: inherit;
+    &.md-peekaboo-active {
+      display: none;
+    }
+  }
+}

ng2-material/components/peekaboo/peekaboo.ts

@@ -0,0 +1,67 @@
+import {Directive} from "angular2/core";
+import {Media, MediaListener} from '../../core/util/media';
+import {DOM} from "angular2/src/platform/dom/dom_adapter";
+import {OnDestroy} from "angular2/core";
+import {debounce} from "../../core/util/util";
+import {Input} from "angular2/core";
+import {CONST} from "angular2/src/facade/lang";
+import {isPresent} from "angular2/src/facade/lang";
+import {Attribute} from "angular2/core";
+
+
+/** Different peekaboo actions to apply when active */
+@CONST()
+export class PeekabooAction {
+  @CONST() static SHOW = 'show';
+  @CONST() static HIDE = 'hide';
+}
+
+/**
+ * @name mdPeekaboo
+ *
+ * @description
+ * The `[md-peekaboo]` directive is an attribute that toggles the visibility of elements based
+ * on the current viewport size and scrollTop.
+ *
+ */
+@Directive({
+  selector: '[md-peekaboo]',
+  inputs: ['breaks'],
+  host: {
+    '[class.md-peekaboo-active]': 'active',
+    '[attr.action]': 'action'
+  }
+})
+export class MdPeekaboo implements OnDestroy {
+  //TODO(jdd): This is just testing hacks, use mapping size->scrollTop
+  @Input() breaks: number = 100;
+
+  @Input() action: string;
+
+  private _active: boolean;
+  get active(): boolean {
+    return this._active;
+  }
+
+  get scrollTop(): number {
+    return window.pageYOffset || document.documentElement.scrollTop;
+  }
+
+  constructor(@Attribute('action') action: string) {
+    this.action = isPresent(action) ? action : PeekabooAction.SHOW;
+    this._active = this._evaluate(this.scrollTop);
+    window.addEventListener('scroll', this._windowScroll);
+  }
+
+  ngOnDestroy(): any {
+    window.removeEventListener('scroll', this._windowScroll);
+  }
+
+  private _evaluate(scrollTop: number): boolean {
+    return scrollTop >= this.breaks;
+  }
+
+  private _windowScroll = (ev: Event) => {
+    this._active = this._evaluate(this.scrollTop);
+  };
+}