Python 3 support

```
Running setup.py egg_info for package gitdb
  Traceback (most recent call last):
    File "<string>", line 16, in <module>
    File "/tmp/pip-build-root/gitdb/setup.py", line 24
      print "Ignored failure when building extensions, pure python modules will be used instead"
```

Just popping this in here as a placeholder, apologies if I missed this elsewhere  Looks like an interesting project!
