refactor(router): improve recognition and generation pipeline

This is a big change. @matsko also deserves much of the credit for the implementation.

Previously, `ComponentInstruction`s held all the state for async components.
Now, we introduce several subclasses for `Instruction` to describe each type of navigation.

BREAKING CHANGE:

Redirects now use the Link DSL syntax. Before:

```
@RouteConfig([
  { path: '/foo', redirectTo: '/bar' },
  { path: '/bar', component: BarCmp }
])
```

After:

```
@RouteConfig([
  { path: '/foo', redirectTo: ['Bar'] },
  { path: '/bar', component: BarCmp, name: 'Bar' }
])
```

BREAKING CHANGE:

This also introduces `useAsDefault` in the RouteConfig, which makes cases like lazy-loading
and encapsulating large routes with sub-routes easier.

Previously, you could use `redirectTo` like this to expand a URL like `/tab` to `/tab/posts`:

@RouteConfig([
  { path: '/tab', redirectTo: '/tab/users' }
  { path: '/tab', component: TabsCmp, name: 'Tab' }
])
AppCmp { ... }

Now the recommended way to handle this is case is to use `useAsDefault` like so:

```
@RouteConfig([
  { path: '/tab', component: TabsCmp, name: 'Tab' }
])
AppCmp { ... }

@RouteConfig([
  { path: '/posts', component: PostsCmp, useAsDefault: true, name: 'Posts' },
  { path: '/users', component: UsersCmp, name: 'Users' }
])
TabsCmp { ... }
```

In the above example, you can write just `['/Tab']` and the route `Users` is automatically selected as a child route.

Closes #4170
Closes #4490
Closes #4694
Closes #5200

Closes #5352

modules/angular2/src/router/router_link.ts

@@ -3,7 +3,7 @@ import {isString} from 'angular2/src/facade/lang';
 
 import {Router} from './router';
 import {Location} from './location';
-import {Instruction, stringifyInstruction} from './instruction';
+import {Instruction} from './instruction';
 
 /**
  * The RouterLink directive lets you link to specific parts of your app.
@@ -61,7 +61,7 @@ export class RouterLink {
     this._routeParams = changes;
     this._navigationInstruction = this._router.generate(this._routeParams);
 
-    var navigationHref = stringifyInstruction(this._navigationInstruction);
+    var navigationHref = this._navigationInstruction.toLinkUrl();
     this.visibleHref = this._location.prepareExternalUrl(navigationHref);
   }