Integrate django-pyscss

It's possible to integrate django-pyscss in a completely backwards-compatible way, but it feels a little gross.  If we were to add the following classes:

```python
# widgy/scss.py
from django_pyscss.scss import DjangoScss
from django_pyscss.compressor import DjangoScssFilter


class WidgyDjangoScss(DjangoScss):
    """
    Provides a backwards-compatible shim for the previously recommended style.
    """
    def get_possible_import_paths(self, path, relative_to=None):
        if path.startswith('widgy_common'):
            path = '/widgy/css/widgy_common.scss'
        return super(WidgyDjangoScss, self).get_possible_import_paths(path, relative_to)


class WidgyDjangoScssFitler(DjangoScssFilter):
    compiler = WidgyDjangoScss()
```

And then tell people to use `widgy.scss.WidgyDjangoScssFilter` instead of `django_pyscss.compressor.DjangoScssFilter`, we could support both the `SCSS_IMPORTS` old-style and the `django-pyscss` new style, however, this would be pretty gross because we wouldn't be able to update the imports for widgy_common.scss because they would break the old-style, so we would be crippling the new style.

I was going to make a pull request to integrate django-pyscss, but I want to know if we should do it in the backwards compatible way or do it in the backwards incompatible clean cut way.