Rename ol.structs.RBush#forEach to ol.structs.RBush#some

See comments in #2186

But I'm not very happy with it: 
`forEachInExtent` should be renamed to `someInExtent` as well as `ol.source.Vector#forEachFeature`, `ol.source.Vector#forEachFeatureAtCoordinate`, `ol.source.Vector#forEachFeatureInExtent` ...

In the meantime I played with the [goog.iter](http://docs.closure-library.googlecode.com/git/namespace_goog_iter.html) package (see https://github.com/fredj/ol3/compare/iterator): this could provide a far flexible and simple system to iterate over the tree.