Introduce extension interfaces for PollingTradeService#getTradeHistory

Resolve #718 by adding 2 interfaces to be optionally implemented by PollingTradeService implementations.

Clients can monitor trade history with code similar to this:

    if(pts instanceof SinceTradeHistoryProvider)
      userTrades = ((SinceTradeHistoryProvider)pts).getTradeHostory(lastTrade);
    else if(pts instanceof LimitedTradeHistoryProvider)
      userTrades = ((LimitedTradeHistoryProvider)pts).getTradeHistory(count);
    else
      userTrades = pts.getTradeHistory()