feat(ionInfiniteScroll): use event system

Fixes #661.

BREAKING CHANGE: The binding for ionInfiniteScroll has changed, as well
as how you finish it.

If you had this code before:

```html
<ion-content on-infinite-scroll="doSomething"></ion-content>
```
```js
function MyCtrl($scope) {
  $scope.doSomething = function(scrollDoneCallback) {
    doSomething();
    scrollDoneCallback();
  };
}
```js

Now, your code should look like this:

```html
<ion-content on-infinite-scroll="doSomething()"></ion-content>
```
``js
function MyCtrl($scope) {
  $scope.doSomething = function() {
    doSomething();
    $scope.$broadcast('scroll.infiniteScrollComplete');
  };
}
```