Use the "controller as" syntax

This PR changes the examples to use the "controller as" syntax with `ng-controller`.

When reading the HTML this syntax makes it explicit where the data comes from. For example, if I have 

```html
<body ng-controller="MainController as crtl">
  …
  …
  <button ngeo-btn class="btn btn-success" ng-model="ctrl.drawPoint.active">Point</button>
```

I know that the `drawPoint` interaction is defined in the `MainController`.

Note that the `map` instance still needs to be defined in the `$scope`. The `map` directive requires it.  I added docs in the map directive file to make this clear.

Please review.