Fix miss import re on windows

Python.exe 2.7.8 on windows

```sh
Traceback (most recent call last):
  File "ninepatch.py", line 305, in <module>
    main()
  File "ninepatch.py", line 295, in main
    result = np.render(parse_size(opts.size))
  File "ninepatch.py", line 268, in parse_size
    return tuple(map(int, re.split("x", sizeS, re.I)))
NameError: global name 're' is not defined
```