refactor(overlays): inject overlay providers

BREAKING CHANGES:

- Overlay components, such as Alert or Modals, should now be created
using its injected provider.
- Overlays now have the `present()` method on the overlay’s instance,
rather than using `nav.present(overlayInstance)`.
- All overlays now present on top of all app content, to include menus.
- Below is an example of the change to `Alert`, but the pattern is the
same for all overlays: ActionSheet, Loading, Modal, Picker, Popover,
Toast

WAS:

```
import { NavController, Alert } from ‘ionic-angular’;

constructor(private nav: NavController) {
}

doAlert() {
  let alert = Alert.create({
    title: 'Alert',
  });
  this.nav.present(alert);
}
```

NOW:

```
import { AlertController } from ‘ionic-angular’;

constructor(private alertCtrl: AlertController) {
}

doAlert() {
  let alert = this.alertCtrl.create({
    title: 'Alert'
  });
  alert.present();
}
```

src/components/nav/nav.ts

@@ -5,7 +5,6 @@ import { Config } from '../../config/config';
 import { Keyboard } from '../../util/keyboard';
 import { isTrueProperty } from '../../util/util';
 import { NavController } from './nav-controller';
-import { NavPortal } from './nav-portal';
 import { ViewController } from './view-controller';
 
 /**
@@ -108,8 +107,10 @@ import { ViewController } from './view-controller';
  */
 @Component({
   selector: 'ion-nav',
-  template: '<div #viewport nav-viewport></div><div class="nav-decor"></div><div nav-portal></div>',
-  directives: [NavPortal],
+  template: `
+    <div #viewport nav-viewport></div>
+    <div class="nav-decor"></div>
+  `,
   encapsulation: ViewEncapsulation.None,
 })
 export class Nav extends NavController implements AfterViewInit {
@@ -194,8 +195,4 @@ export class Nav extends NavController implements AfterViewInit {
     this._sbEnabled = isTrueProperty(val);
   }
 
-  @ViewChild(NavPortal)
-  private set _np(val: NavPortal) {
-    this.setPortal(val);
-  }
 }