Added support in IECoreNuke::DeepImageReader for ZBack channels.
Updated the SConstruct to enable the use of Deep EXR with IECoreNuke.
Added a test case that checks that the IECoreNuke::DeepImageReader correctly adds the ZBack channel if one does not exist.
Merge pull request #243 from goddardl/NukeDeepReaderBack

Added support in IECoreNuke::DeepImageReader for ZBack channels