Add ol.geom.Polygon.fromExtent

@tonio was [positive](https://github.com/elemoine/ol3/commit/f92b0073d89847bb0ee09b0fd261b851ff6b8657#commitcomment-8115108) about it, so here it is. Other suggestions for the function name are welcome.
You might as well orient these clockwise so we don't have to duplicate the array for `orientedFlatCoordinates_`.