Provide an API to refresh a source and reload its data in an associated
layer, as for example already loaded tiles in a ol.source.Tile source
are not refreshed. Additionally a test for the new
ol.source.Source.refresh() function is provided.
Currently internal state is not cleaned up, e.g. already loaded tiles
in a `ol.source.Tile` source are still cached and not reloaded at all.
github issue #4867 (https://github.com/openlayers/ol3/issues/4867)
