Updated django-node dependency to latest.
API change.

`django_webpack.models.WebpackBundle.output` is now `django_webpack.models.WebpackBundle.path_to_output`
`django_webpack.models.WebpackBundle.get_output` is now `django_webpack.models.WebpackBundle.get_path_to_output`
Added an optional in-memory cache, triggered by the setting `DJANGO_REACT['CACHE']`.
Adding documentation.
Bumping the version to 0.2.0.
Updating the changelog.