How to rename directories?

This is an example:

```
$ mkdir 1
$ cd 1
$ touch {1,2,3,4,5}.txt
$ mkdir {6,7,8,9}
$ cp *txt 6
$ repren -n --renames --from "^" --to "aaa_" [6789]
Dry run: No files will be changed
Using 1 patterns:
'^' -> 'aaa_'
Found 5 files in: 6, 7, 8, 9
- rename: 6/4.txt -> aaa_6/4.txt
- rename: 6/1.txt -> aaa_6/1.txt
- rename: 6/3.txt -> aaa_6/3.txt
- rename: 6/5.txt -> aaa_6/5.txt
- rename: 6/2.txt -> aaa_6/2.txt
Read 5 files (0 chars), found 0 matches (0 skipped due to overlaps)
Dry run: Would have changed 5 files (0 rewritten and 5 renamed)

$ repren --renames --from "^" --to "aaa_" [6789]
Using 1 patterns:
'^' -> 'aaa_'
Found 5 files in: 6, 7, 8, 9
Traceback (most recent call last):
File "/home/alexey/bin/repren", line 531, in
dry_run=options.dry_run)
File "/home/alexey/bin/repren", line 406, in rewrite_files
rewrite_file(path, patterns, do_renames=do_renames, do_contents=do_contents, by_line=by_line, dry_run=dry_run)
File "/home/alexey/bin/repren", line 372, in rewrite_file
counts = transform_file(transform, path, dest_path, by_line=by_line, dry_run=dry_run)
File "/home/alexey/bin/repren", line 357, in transform_file
move_file(source_path, dest_path, clobber=False)
File "/home/alexey/bin/repren", line 299, in move_file
shutil.move(source_path, dest_path)
File "/usr/lib/python2.7/shutil.py", line 302, in move
copy2(src, real_dst)
File "/usr/lib/python2.7/shutil.py", line 130, in copy2
copyfile(src, dst)
File "/usr/lib/python2.7/shutil.py", line 83, in copyfile
with open(dst, 'wb') as fdst:
IOError: [Errno 2] No such file or directory: 'aaa_6/4.txt'
```

So it is a **bug** plus feature request (rename folders only, or at least rename them like files)