Django-dashing

- Fix Rickshaw link
- Fix is_authenticated not a callable after django 2
- Support change color for list widget

- Fix bug when many widgets try to load the same dependency
- Update test suite

- Add option to set a background icon in the number widget
- Add setting to add google maps API key for the map widget
- Bug fixes

- Ability to set permission for each widget
- Bug fixes

- Add KnobWidget and MapWidget python classes
- Fix several bugs
- Remove backward compatibility code

- Add `getDashboards` method in **DashboardSet** object and `getWidgets` method in **Dashboard** object
- Improve resource loading adding some minified libraries and using preloadjs to load the libs requires by the widgets
- Fix some design issues
- Add Knob and Map plugins [docs](https://github.com/talpor/django-dashing/pull/20)