improved deps.py script. see issue 542 for details

This is a handy script, but the original documentation and usage is little confusing. After the discussion with original writer [#542](https://github.com/spotify/luigi/issues/542), I made the following changes.

* revise the script usage to 
```
PYTHONPATH=$PYTHONPATH:/path/to/your/luigi/tasks bin/deps.py \
--module my.tasks  MyDownstreamTask
--downstream_task_param1 123456
[--upstream MyUpstreamTask]
```
* list all upstream dependencies if no specific upstream task are given.
* display output file of each task for easy debug and re-run. sample output as below:
```
   TASK: DBTask1(date=2014-11-19)
                       : [DB] db_name:table_name
   TASK: RemoteTask2(date=2014-11-19)
                       : [SSH] host_name: file_name
   TASK: S3Task3(date=2014-11-19)
                       : [S3] s3://path/to/output/file
```