The writeFeatures method should always return a string

This PR fixes  #2102

All writeFeatures, writeFeature and writeGeometry methods now return a string. The respective methods such as writeFeaturesObject (JSON) and writeFeaturesNode (XML) have been marked api but not stable as yet.
good point will fix on Monday

Sent from my iPhone

> On 6 dec. 2014, at 20:21, Tim Schaub <notifications@github.com> wrote:
> 
> In examples/gpx.js:
> 
> > @@ -105,7 +105,7 @@ if ('download' in exportGPXElement) {
> >          clone.getGeometry().transform(projection, 'EPSG:4326');
> >          features.push(clone);
> >        });
> > -      var node = new ol.format.GPX().writeFeatures(features);
> > +      var node = new ol.format.GPX().writeFeaturesNode(features);
> Shouldn't this be simplified to use writeFeatures instead of creating a node and then serializing with XMLSerializer below?
> 
> —
> Reply to this email directly or view it on GitHub.
>