Make the jardiff.py fully cross-platform

Make the jardiff.py fully cross-platform. Thanks a lot for help me, the script is working on Windows as well as on UNIX platforms :smile:.

I do not know how to program in Python, i'm a .Net guy, some things can get better :smile: in code

In Windows requires Cygwin. After I send another pull request with information about its installation and required package.

Thanks a lot.
Could you merge this:
```python
for i in range(len(chunk)):
  chunk[i] = os.path.relpath(chunk[i], unjar).replace(".class", "")
```
in [_chunks](https://github.com/JakeWharton/jardiff/blob/master/jardiff.py#L31) method instead of ```yield items[i:i + chunk_size]```?
I'm sorry, I'm a .Net guy, I don't know how to program in Python, merging the loop above in ```_chunks``` method will make the code faster.