Dash

Fixed
- [387](https://github.com/plotly/dash/pull/387) Add `Cache-Control` headers to files served by `Dash.serve_component_suites`, and time modified query string to collected components suites resources.
- [394](https://github.com/plotly/dash/pull/394) Add `InvalidResourceError` error and a Flask error handler so unregistered paths in `serve_component_suites` return a 404 instead of 500.

Fixed
- [374](https://github.com/plotly/dash/pull/374) Fix `get_asset_url` with a different `assets_url_path`.

Fixed
- Set `url_base_pathname` to `None` in `Dash.__init__`. Fix [364](https://github.com/plotly/dash/issues/364)

Added
- `Dash.get_asset_url` will give the prefixed url for the asset file.

Fixed
- [351](https://github.com/plotly/dash/pull/351) Prefix assets files with `requests_pathname_prefix`.

Fixed
- [343](https://github.com/plotly/dash/pull/343) Only create the assets blueprint once for apps that provide the same flask instance to multiple dash instances.

Fixed
- [336](https://github.com/plotly/dash/pull/336) Fix bug in `_validate_layout` which would not let a user set `app.layout` to be a function that returns a layout [(fixes #334)](https://github.com/plotly/dash/issues/334).