docs(README): add whitespace
feat(ionRefresher): allow custom text & icons

Closes #760

BREAKING CHANGE: on-refresh and on-refresh-opening are no longer on the
ion-content directive.  They are on the ion-refresher. In addition,
on-refresh-opening has been renamed to on-pulling.

Change your code from this:

```html
<ion-content on-refresh="onRefresh()"
  on-refresh-opening="onRefreshOpening()">
  <ion-refresher></ion-refresher>
</ion-content>
```

To this:

```html
<ion-content>
  <ion-refresher on-refresh="onRefresh()"
    on-pulling="onRefreshOpening()">
  </ion-refresher>
</ion-content>
```
docs(ionContent): update to match new ionRefresher