coerce DDRFile.path to be relative to entity; DDRFile.path_abs

We want file paths in entity.json to be relative to the entity
rather than absolute (absolute paths will break when the collection
is moved).  At the same time, we need absolute path for certain
things during runtime.
use DDRFile.path_abs for thumbnail
adjusted logging during entity-file-add

Added more detail (esp file paths) in some cases.
Less detail in some others.
A little more consistency in use of case.
updated update.sh script

- Must now be run as root.
- Assumes that ddr-local, ddr-cmdln installed in /usr/local/src.
- Updates ddr-cmd as well as ddr-local.