BLD Release 0.9.4

Many changes, full ChangeLog:
- Add CustomHash wrapper to set __jug_hash__
- Print traceback on import error
- Exit when no progress is made even with barrier
- Use Tasklets for better jug.mapreduce.map
- Use Ipython debugger if available (patch by Alex Ford)
- Faster --aggressive-unload
- Add currymap() function