IECoreRI::Renderer camera improvements

This gives a big overhaul to our camera support in IECoreRI::Renderer, which was pretty woeful before. It removes a bunch of deprecated options, and adds support for multiple named cameras, camera motion blur and camera edits.

This is a prerequisite for completing https://github.com/ImageEngine/gaffer/issues/190 - I have the Gaffer side for this almost ready - I should be submitting that part for review tomorrow all being well.