Move migrate_task_status.py to CLI paster command

[KrzysztofMadejski]

@davidread any cons to this? Why hasn't this feature been coded as paster command in the first place?

[davidread]

I did it this way because I find stand-alone scripts more flexible and a little easier to write. I think paster is good for day-to-day and official commands though, so I'd be happy if you did move this in, if you think it would benefit from this.