OS X: Restore explicit Python 3 executable listing

The Python 3 binaries installed by MacPorts aren't readily picked up by
'which'.

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>
Begin implementation of other-namespace elements attached to FileObjects
Add further property documentation
Add externals interface to DFXMLObject and VolumeObject
Add unit test for volume externals
Log changes
Objects.py: Allow iterparse() to run a user-specified Fiwalk build

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>