Support on_change callback functions and if custom key is passed use …

…it in session state (#241)

* Support on_change callback functions and if custom key is passed use it in session state

* Use hash key

* Update tests for indentation

* Dedent

Update setup.py

Update setup.py

Update setup.py (#226)

Update setup.py

Update setup.py

Fix error where default_css was missing (#201)

* Fix error where default_css was missing
* Ignore elements where there is no special css or js to load
* Add test to check for regression on geojson popup
* Fix selector for other popup test
* Bump version to 0.21.1

* Fix a few tests

* Debugging, check for failed responses

* Try switching back to old locator

* Try both selectors

Update setup.py

Update setup.py

Update setup.py