Merge pull request #1 from raphaelsc/master

Implement shrinker example function
Merge pull request #2 from raphaelsc/master

Add todo comment
Remove empty directories in "make clean"

Signed-off-by: Don Marti <dmarti@cloudius-systems.com>
Extend README, add some TODO items

Signed-off-by: Don Marti <dmarti@cloudius-systems.com>
Add -depth option to make clean target

Fix deletion of nested empty directories

Signed-off-by: Don Marti <dmarti@cloudius-systems.com>
Change shrinker to release oldest data first

Removing prev also makes the code simpler.
Dealing with concurrency