Major refactor.

Now relying on `django_webpack` for the bundling functionality.
Renamed `django_react.utils` to `django_react.react`.
`ReactComponent.render` is now `ReactComponent.render_to_string`
`ReactComponent.render_static` is now `ReactComponent.render_to_static_markup`
`ReactComponent.get_component_variable` is now `ReactComponent.get_library`, inline with web pack
Moved the web pack configuration into the ReactComponent class.