Restore select interaction behaviour

This PR restores the select interaction behaviour that was added in #1871 but broken by #1877. This fixes #1890.

@tschaub, this also updates the modify-features example [as requested](https://github.com/openlayers/ol3/issues/1890#issuecomment-38838238). I'm reasonably sure that I haven't broken anything, but I'd appreciate a double-check (i.e. real test) before merging if this is going to go into beta 3.

cc @elemoine @tonio