chore(main): add isDefined alias for angular.isDefined
fix(ionCheckbox): make ng-checked and ng-change work

Closes #1349. Closes #1361

BREAKING CHANGE: ion-checkbox no longer has an isolate scope.

This will break your checkbox only if you were relying upon the
checkbox having an isolate scope: if you were referencing
`$parent.value` as the ng-disabled attribute, for example.

Change your code from this:

```html
<ion-checkbox ng-disabled="{{$parent.isDisabled}}"></ion-checkbox>
```

To this:

```html
<ion-checkbox ng-disabled="{{isDisabled}}"></ion-checkbox>
```