Added ScenePlug::pathToString() method.

This mirrors the existing stringToPath() method.

Also fixed ostream output of empty paths.
Added outputLight() method to RendererAlgo.h.
Added test case for disabled GafferScene::Lights.

While hunting another bug it was necessary to check that disabled lights don't output a light set. It turns out that they don't and the problem is elsewhere, but more tests are always good.
Fixed bug in DependencyNode dirty propagation order.

This ensures that dirtiness is only signalled for a plug after it has been signalled for all the plugs it depends on and all its children.
Implemented light add/remove/hide/show for IPR.

Fixes #874.
Merge pull request #880 from johnhaddon/lightIPR

Implemented light add/remove/show/hide in IPR renderer