feat(debug): replace DebugElement with new Debug DOM

Now, using `ng.probe(element)` in the browser console returns
a DebugElement when in dev mode.

`ComponentFixture#debugElement` also returns a new DebugElement.

Breaking Change:

This is a breaking change for unit tests. The API for the DebugElement
has changed. Now, there is a DebugElement or DebugNode for every node
in the DOM, not only nodes with an ElementRef. `componentViewChildren` is
removed, and `childNodes` is a list of ElementNodes corresponding to every
child in the DOM. `query` no longer takes a scope parameter, since
the entire rendered DOM is included in the `childNodes`.

Before:

```
componentFixture.debugElement.componentViewChildren[0];
```

After
```
// Depending on the DOM structure of your component, the
// index may have changed or the first component child
// may be a sub-child.
componentFixture.debugElement.children[0];
```

Before:

```
debugElement.query(By.css('div'), Scope.all());
```

After:

```
debugElement.query(By.css('div'));
```

Before:

```
componentFixture.debugElement.elementRef;
```

After:

```
componentFixture.elementRef;
```

modules/angular2/examples/core/debug/ts/debug_element/debug_element.ts

@@ -1,16 +1,8 @@
-import {DebugElement, Scope} from 'angular2/core';
+import {DebugElement} from 'angular2/core';
 
 var debugElement: DebugElement;
 var predicate;
 
 // #docregion scope_all
-debugElement.query(predicate, Scope.all);
-// #enddocregion
-
-// #docregion scope_light
-debugElement.query(predicate, Scope.light);
-// #enddocregion
-
-// #docregion scope_view
-debugElement.query(predicate, Scope.view);
+debugElement.query(predicate);
 // #enddocregion