refactor(refresher): allow refresher content customization

Breaking Change:

## Refresher:

- `<ion-refresher>` now takes a child `<ion-refresher-content>`
component.
- Custom refresh content components can now be replaced for Ionic's
default refresher content.
- Properties `pullingIcon`, `pullingText` and `refreshingText` have
been moved to the `<ion-refresher-content>` component.
- `spinner` property has been renamed to `refreshingSpinner` and now
goes on the `<ion-refresher-content>` component.
- `refreshingIcon` property is no longer an input, but instead
`refreshingSpinner` should be used.

Was:

```
<ion-refresher (refresh)="doRefresh($event)"
pullingIcon="arrow-dropdown">
</ion-refresher>
```

Now:

```
<ion-refresher (refresh)="doRefresh($event)">
  <ion-refresher-content
pullingIcon="arrow-dropdown"></ion-refresher-content>
</ion-refresher>
```

ionic/components/app/structure.scss

@@ -9,6 +9,9 @@ $z-index-menu-backdrop:          79;
 $z-index-overlay:                1000;
 $z-index-click-block:            9999;
 
+$z-index-scroll-content:         1;
+$z-index-refresher:              0;
+
 $z-index-navbar-section:         10;
 
 $z-index-toolbar:                10;
@@ -129,6 +132,7 @@ ion-content {
 
 scroll-content {
   position: absolute;
+  z-index: $z-index-scroll-content;
   top: 0;
   right: 0;
   bottom: 0;