`with`

It would be cool if I could write:

```
        with ProgressBar(widgets=widgets, maxval=os.stat(fname).st_size) as pbar:
            fp = open(fname, 'rb')
            fp = ProgressFile(fp, pbar.update)
            client.uploadFile(dataset, fp, posixpath.join(dir, file))
```

With `pbar.start()` and `pbar.finish()` sequestered into `pbar.__enter__` and `pbar.__exit__` respectively.