Gaffer: Add support for Reference node fileName search paths
Gaffer: Add support for Reference node fileName search paths

Reference::load() resolves fileName by a IECore::SearchPath find() in a
GAFFER_REFERENCE_PATHS environment variable.

johnhaddon/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,appleseedhq/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,boberfly/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,hradec/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,boberfly/gaffer,ImageEngine/gaffer,hradec/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,boberfly/gaffer,hradec/gaffer,hradec/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,hradec/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,lucienfostier/gaffer,boberfly/gaffer