Expose list of connections as an attribute of WebSocketServer

[aaugustin]

It should be as simple as::

@property
def connections(self):
    return self.handlers.keys()

[aaugustin]

Follow-up task: update documentation to take advantage of this new API.

[aaugustin]

Documentation was updated, notably in 61a6a7a99534e2522773476fc72ca615f5a6ea97 and 667e418ae474235c2bc26024df11c8e1d573d27a.