update changelog and todo
bugfix on Makefile where install fails on cp -i for config files
Run backup hook command even if the backup fails

The exit code of the backup is available to the hook in the
PITRERY_EXIT_CODE env variable.

This change makes the post backup hook scripts not comptatible with
version 1.6 and older.
update docs to add info on the exit code available to the post backup hook
update TODO