Add callback for abode connection status

~~This is a work in progress but I wanted to make this PR to be able to get some feedback.~~ I'm attempting to add support for setting the entity available property in Home Assistant for abode devices/automations. Basically, when available is True, the entities are available to use and when False, the entities are greyed out in the HA UI and can't be used. Here's an overview on my initial thought at implementation:

Use the existing _on_socket_connect and _on_socket_disconnect callbacks in the AbodeEventController class in event_controller.py. These are called when the web socket connection to the Abode server is connected and disconnected.
Create a new property self._is_connected on the AbodeEventController that is set to False by default. This will True when _on_socket_connect is called and False when _on_socket_disconnect is called. A property method is_connected is also created to return self._is_connected for accessing outside the class.
Create a new method add_connection_status_callback that appends callbacks to a newly added empty list self._connection_status_callback.

From the Home Assistant side, I'm simply calling add_connection_status_callback for each instance of AbodeDevice (in the abode HA integration). This in turn adds a simple callback method:

    def _update_connection_status(self):
        """Update the device available property."""
        self._available = self._data.abode.events.connected
        self.schedule_update_ha_state()

When _on_socket_connect is called, it will call the callback above for every Abode device in Home Assistant which will set the self._available property based on the self._is_connected boolean.

Everything is working as expected during testing but I'd like to get some other opinions.

MisterWil / abodepy

Add callback for abode connection status #67