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/ion.ts

@@ -1,6 +1,4 @@
 import {ElementRef} from 'angular2/core';
-import {Config} from '../config/config';
-import {isArray} from '../util';
 import * as dom from '../util/dom';
 
 let ids:number = 0;
@@ -17,24 +15,30 @@ export class Ion {
     this._id = 'i' + ids++;
   }
 
-  getElementRef() {
+  getElementRef(): ElementRef {
     return this.elementRef;
   }
 
-  getNativeElement() {
+  getNativeElement(): any {
     return this.elementRef.nativeElement;
   }
 
-  getDimensions() {
+  getDimensions(): {
+    width: number, height: number, left: number, top: number
+  } {
     return dom.getDimensions(this.elementRef.nativeElement, this._id);
   }
 
-  width() {
+  width(): number {
     return dom.getDimensions(this.elementRef.nativeElement, this._id).width;
   }
 
-  height() {
+  height(): number {
     return dom.getDimensions(this.elementRef.nativeElement, this._id).height;
   }
 
+  ngOnDestroy() {
+    dom.clearDimensions(this._id);
+  }
+
 }