Move attribution constants to constructors

Currently, some attribution constants are added to the `ol.source` object (e.g. [`ol.source.STAMEN_ATTRIBUTIONS`](https://github.com/openlayers/ol3/blob/82a158bdd20fbf39f82eeaf35051f7fb37411df2/src/ol/source/stamensource.js#L82)) and others are added to the source constructors (e.g. [`ol.source.OSM.ATTRIBUTIONS`](https://github.com/openlayers/ol3/blob/82a158bdd20fbf39f82eeaf35051f7fb37411df2/src/ol/source/osmsource.js#L68)).  Given the way that provide/require checking works, it would be more consistent to make all attributions properties of the provided constructors.