ProgressBar() module to display output to a console or iPython Re: issue#2658

Modified ProgressBar() module to display output to a console or iPython notebook, depending on additional 'interactive' kwarg.

Description of the issue and possible fix here: http://nbviewer.ipython.org/gist/astrocaribe/398d04434430c3f44617

the above two lines needn't be duplicated.  Instead this could be written something like:

```
if not hasattr(self, '_pb'):  # could we maybe call this self._widget instead?
    self._pb = widgets.FloatProgressWidget()
    display(self._pb)
    value = 0

self._pb.value = 0
self._pb.description = ...
```

For the description, please use string formatting syntax instead.  You can see how the label is formatted for the console version starting around here:

https://github.com/astrocaribe/astropy/blob/issue_2658/astropy/utils/console.py#L547

I think it would probably be possible to separate that code into a separate utility method shared by both `_update` methods.  Let me know if you need any help with that, and thanks again for doing this!