clean up temporary directories please

running

```
TMPDIR=$PWD/TEMP GIT_PYTHON_TEST_GIT_REPO_BASE=/tmp/GitPython/.git nosetests -s -v git/test
```

leaves

```
$> ls -R TEMP 
TEMP:
tmpcemxMt/  tmpfSfjlH/

TEMP/tmpcemxMt:

TEMP/tmpfSfjlH:
```

behind
