Installing from PyPI doesn't install include Django management command

Only the 'whitenoise' package was given in setup.py (and not also whitenoise.management and whitenoise.management.commands) which led to these packages not being included in site-packages after installing from PyPI. This patch fixes that.

For reference, installing v0.12:

```bash
$ pip install --find-links ./lib --no-index whitenoise==0.12
Downloading/unpacking whitenoise==0.12
Successfully installed whitenoise
$ tree venv/lib/python3.3/site-packages/whitenoise
venv/lib/python3.3/site-packages/whitenoise
├── __init__.py
├── base.py
├── django.py
└── gzip.py
```

Installing fixed version (v0.13):

```bash
$ pip install --find-links ./lib --no-index whitenoise==0.13
Downloading/unpacking whitenoise==0.13
Successfully installed whitenoise
$ tree venv/lib/python3.3/site-packages/whitenoise
venv/lib/python3.3/site-packages/whitenoise
├── __init__.py
├── base.py
├── django.py
├── gzip.py
└── management
    ├── __init__.py
    └── commands
        ├── __init__.py
        └── gzipstatic.py
```