API refactor and general cleanup.

The primary goal was to decouple the util methods (bundle and render) from the ReactComponent API, so as to make the bundling and rendering process more configurable from a model level.

`django_react.exceptions.ReactComponentSourceFileNotFound` is now `django_react.exceptions.SourceFileNotFound`
`django_react.exceptions.ReactComponentRenderToStringException` is now `django_react.exceptions.RenderException`
`django_react.exceptions.ReactComponentBundleException` is now `django_react.exceptions.BundleException`
`django_react.models.ReactComponent` now has additional methods: `generate_path_to_bundled_source`, `write_bundled_source_file`, `generate_bundled_source_file`, `get_rel_path_to_bundled_source`, and `get_url_to_bundled_source`.
`django_react.utils.bundle` no longer accepts a `ReactComponent` as an argument, it now takes `entry` and `library`.
`django_react.utils.render` no longer accepts a `ReactComponent` as an argument, it now takes `path_to_source`, `serialised_props`, and `to_static_markup`.
`django_react/render.js` no longer accepts the `--path-to-component` argument, instead it takes `--path-to-source`.
Updating the PyPI description text.
Bumping the version to 0.3.0.