implement ready signal (connected, all data has been downloaded, trading can begin)
added new signals to OrderBook (experimental, please test and report problems):
signal_own_added           = Signal()  # order was added (pending)
signal_own_removed         = Signal()  # order has been removed
signal_own_opened          = Signal()  # order status went to "open"
signal_own_volume          = Signal()  # order volume changed (partial fill)