Fixed RendererAlgo outputScene() to include coordinate systems.
Added global mode to GafferScene::Attributes node.

This places the attributes in the globals (with "attribute:" prefixes) rather than at locations in the scene. This is for use in setting up "render globals" networks where in fact some of the values being specified are actually renderer attributes rather than options (maximum ray depth in 3delight for instance).
Updated SceneInspector to display global attributes.
Updated render nodes to support global attributes.
Added test demonstrating ArrayPlug working with python child plugs.
Fixed bug in Attributes node.

Also added some comments to sort things out so it's harder to get wrong in the future.
Added global attribute support to SceneProcedural.

Fixes #964.
Merge pull request #976 from johnhaddon/globalAttributes

Global attributes
Updating version and changes.
Updating version and changes.
Merge branch 'master' of https://github.com/ImageEngine/gaffer

Conflicts:
	Changes
Reordering dependency includes at IE.

Now that we're using OpenEXR 2.1.0 across the board, we need those includes to come before the app specific ones, in case some app (Houdini 13) is using a different OpenEXR 2+ version, but did build with a versioned namespace...