Add ol.Feature#clone

This PR suggests adding a clone method to ol.Feature. Please review.

My use-case: I need to serialize features in a different projection. Without a clone method it looks like this:

```js
var features = [];
vectorSource.forEachFeature(function(feature) {
  var clone = new ol.Feature();
  clone.setId(feature.getId());
  clone.setValues(feature.getProperties());
  clone.setGeometryName(feature.getGeometryName());
  var geometryClone = feature.getGeometry().clone();
  clone.setGeometry(geometryClone);
  var style = feature.getStyle();
  if (style) {
    clone.setStyle(style);
  }
  features.push(clone);
});
var serialized = format.writeFeatures(features);
```

With a clone method it looks like this:

```js
var feature = [];
vectorSource.forEachFeature(function(feature) {
  var clone = feature.clone();
  clone.setId(feature.getId());  // clone does not set the id
  features.push(clone);
});
var serialized = format.writeFeatures(features);
```
A feature may not have a geometry!