rendernode.py 1.15: tweaked timestamps formatting.
rendermanager.py 1.3: many changes.

- fixed bug in dependency tracking (project files being rendered were not considered when computing dependencies).
- allow to specify a target directory for rendered frames.
- don't crash when the rendered frame files are locked.
- tweaked timestamps formatting.
Merge branch 'master' of github.com:jupiter-jazz/appleseed