The feature is planned to make it into Cycles/Blender at some point. It probably needs a bit of polishing and potentionally a few issues fixed first (I believe the patch doesn't work with adaptive sampling, not 100% sure).
As for implementing it into Blender yourself, you can take the patch and apply it to your own build of Blender. However, I believe some manual work may be required to work with Blender in it's current state. Juan Gea has a build of Blender with this feature built in. You can find it over on GraphicAll