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/select/select.ts

@@ -3,6 +3,7 @@ import { NG_VALUE_ACCESSOR } from '@angular/common';
 
 import { ActionSheet } from '../action-sheet/action-sheet';
 import { Alert } from '../alert/alert';
+import { App } from '../app/app';
 import { Form } from '../../util/form';
 import { isBlank, isCheckedProperty, isTrueProperty, merge } from '../../util/util';
 import { Item } from '../item/item';
@@ -192,6 +193,7 @@ export class Select {
   @Output() ionCancel: EventEmitter<any> = new EventEmitter();
 
   constructor(
+    private _app: App,
     private _form: Form,
     private _elementRef: ElementRef,
     private _renderer: Renderer,
@@ -205,10 +207,6 @@ export class Select {
       this._labelId = 'lbl-' + _item.id;
       this._item.setCssClass('item-select', true);
     }
-
-    if (!_nav) {
-      console.error('parent <ion-nav> required for <ion-select>');
-    }
   }
 
   @HostListener('click', ['$event'])
@@ -279,7 +277,7 @@ export class Select {
       }));
       alertOptions.cssClass = 'select-action-sheet';
 
-      overlay = ActionSheet.create(alertOptions);
+      overlay = new ActionSheet(this._app, alertOptions);
 
     } else {
       // default to use the alert interface
@@ -297,7 +295,7 @@ export class Select {
       });
 
       // create the alert instance from our built up alertOptions
-      overlay = Alert.create(alertOptions);
+      overlay = new Alert(this._app, alertOptions);
 
       if (this._multi) {
         // use checkboxes
@@ -318,7 +316,7 @@ export class Select {
 
     }
 
-    this._nav.present(overlay, alertOptions);
+    overlay.present(alertOptions);
 
     this._isOpen = true;
     overlay.onDismiss(() => {