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/picker/picker-options.ts

@@ -0,0 +1,27 @@
+
+export interface PickerOptions {
+  buttons?: any[];
+  columns?: PickerColumn[];
+  cssClass?: string;
+  enableBackdropDismiss?: boolean;
+}
+
+export interface PickerColumn {
+  name?: string;
+  align?: string;
+  selectedIndex?: number;
+  prefix?: string;
+  suffix?: string;
+  options?: PickerColumnOption[];
+  cssClass?: string;
+  columnWidth?: string;
+  prefixWidth?: string;
+  suffixWidth?: string;
+  optionsWidth?: string;
+}
+
+export interface PickerColumnOption {
+  text?: string;
+  value?: any;
+  disabled?: boolean;
+}