Make it possible to set map target after map creation

E.g.

```
var map = new ol.Map({
  layers: [
    new ol.layer.TileLayer({
      source: new ol.source.OSM()
    })
  ],
  view: new ol.View2D({
    center: [0, 0],
    zoom: 0
  })
});

map.setTarget(document.getElementById('map'));
```

The PR includes a "teleport map" example demonstrating the feature. 

Refs #536.