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Improvements to the javascript API:
* Performing reconnection attempts when the first connection (on instantiation) fails.
* Adding the 'close()' method to enable closing the connection explicitly. When the connection is
closed calling this method, there will be no reconnection attempts. In order to connect again,
the client must be re-instantiated.
* Adding 'connecting' and 'disconnected' callback options. The first is fired right before the
Websocket is being instantiated, while tha last is fired after connection is closed.
* Adding the following methods to check websocket status: ``is_connecting()``, ``is_connected()``,
``is_closing()``, ``is_closed()``.
* Replaced ``STATIC_URL`` against ``{% static %}`` in all templates.
* Keep track on opened websockets.