Projection can be defined as string, witch allow to define custom projec...

Projection can be defined as string, witch allow to define custom projection with ol.proj.addProjection like this

	var projection = new ol.proj.Projection({
		code: 'EPSG:31370',
	  // The extent is used to determine zoom level 0. Recommended values for a
	  // projection's validity extent can be found at http://epsg.io/.
	  extent: [42000.0, 19999.75, 296000.0, 168000.0],
	  units: 'm'
	});
	ol.proj.addProjection(projection);

By the way, i removed code for EPSG:4326 and EPSG:3857 because they are already defined by default in ol3 http://openlayers.org/en/master/apidoc/ol.proj.Projection.html.

Regards