GafferSeed

appleseed (http://appleseedhq.net/) support for Gaffer.
Works with appleseed's master branch.
Almost all of it is new code. The only changes to Gaffer itself are in SConstruct and startup/gui/menus.py

Minimal instructions:
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- $APPLESEED must be defined and point to appleseed's directory.
- appleseed's binary dir must be on the $PATH.
- appleseed's lib dir must be on the linker path.
- appleseed's python module is only needed to run IECoreAppleseed tests (inside Cortex).
- Uses OSL shaders. They are found using the $APPLESEED_SEARCHPATH env var.
- There's a basic OSL shaders lib (work in progress) for appleseed that can be used in Gaffer here: https://github.com/appleseedhq/appleseed.shaders.
- Only supports rgb and rgba outputs, interactive, batch/exr and batch/png.

`_parseEntitiesMetata()` feels very much like a private implementation detail, and I think we discussed offline that at some point the same information might be available more directly via the appleseed python module. I think it would be best if this call was made automatically inside the new `AppleseedLight.loadShader()` method. That would let us remove the dependency on the UI module from the test module.