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/view-controller.ts

@@ -1,9 +1,10 @@
-import { ChangeDetectorRef, EventEmitter, ElementRef, Output, Renderer } from '@angular/core';
+import { ChangeDetectorRef, ElementRef, EventEmitter, Output, Renderer } from '@angular/core';
 
 import { Footer, Header } from '../toolbar/toolbar';
 import { isPresent, merge } from '../../util/util';
 import { Navbar } from '../navbar/navbar';
-import { NavController, NavOptions } from './nav-controller';
+import { NavController } from './nav-controller';
+import { NavOptions } from './nav-options';
 import { NavParams } from './nav-params';
 
 
@@ -79,11 +80,6 @@ export class ViewController {
    */
   isOverlay: boolean = false;
 
-  /**
-   * @private
-   */
-  usePortal: boolean = false;
-
   /**
    * @private
    */