Added a space plug to GafferScene.Transform.

This allows the user to specify whether the transform is applied in world space (the previous behaviour and the default) or object space.
Removed unnecessary forward declaration from Transform2DPlug.h

It dates back to a time when Transform2DPlug introduced a dependency on GafferImage within Gaffer, which it should never have done. That has been long since fixed, but it seems we didn't remove the forward declaration when we made the fix.
Added pivot to TransformPlug.

Also added optimised plug accessors (getting by index rather than by name).

Fixes #156.
Merge pull request #711 from johnhaddon/transformImprovements

Transform improvements