Major refactor.

Renamed `django_react.exceptions.PropSerialisationError` to `django_react.exceptions.PropSerializationError`.
Rolled the bundling functionality out into a more easily overridable interface. You can now define a `bundle` attribute on `ReactComponent` inheritors which should be an extended `django_webpack.models.WebpackBundle`.
Renamed the following attributes on `ReactComponent`:
- `entry` is now `source`
- `library` is now `variable`
Renamed the `get_library` method on `ReactComponent` to `get_variable`
Removed the following methods on `ReactComponent`:
- `get_serialised_props_hash`
- `get_component_id`
- `get_react_variable`
- `get_component_name`
- `has_props`
The render_* methods now use standard Django templates where possible.
Removed the `render_to_string` and `render_to_static_markup` methods from `django_react.react`. In their place, use `django_react.react.render`.
The react external can now be configured on a per-bundle basis, or globally by using the `DJANGO_REACT['REACT_EXTERNAL']` setting.
Updated django-node and django-webpack dependencies to the latest.
Added a test suite and harness.
Added basic documentation.