Fix multiple Smarkets instances

If you have more than one client, their Callback objects are the same.

    >>> import smarkets.clients
    >>> a = smarkets.clients.Smarkets('_')
    >>> b = smarkets.clients.Smarkets('_')
    >>> def h(msg): print repr(("client a received", msg))
    ... 
    >>> a.add_handler('seto.http_found', h)
    
    # pretend b is connected and got a message...
    >>> b.callbacks['seto.http_found']('example_message')
    ('client a received', 'example_message')