Bump version to 0.16.1
Add a parameter for controlling pithcat's umask

Introduce the PITHCAT_UMASK option. This can be used to set the file
mode mask of the pithcat process.
helper: Exclude all tasks if no properties defined

If no image properties are defined, then do not run any configuration
task. Prior to this commit, the system would fail if no image
properties were present.
Update ChangeLog and version files for v0.16.1