Add a generic "control" directive

This PR adds an generic "control" directive. The directive is responsible for adding a user-provided control to a DOM element in the page.

Usage:

```html
<div ngeo-control="createControl"></div>
```

`createControl` is the name of a scope property referencing a function that creates the control instance.

In the controller:

```js
  $scope['createControl'] = function(target) {
    return new ol.control.MousePosition({
      className: 'mouse-position',
      target: target
    });
  };
```

If people agree with the approach I'll write basic unit tests before merging.

Fixes #59 #55.