pyarb: fix bugs with Decimal; replace btceapi.min_orders with tradeapi.GetMinAmount()
pyarb: fix bugs with Decimal; replace btceapi.min_orders with tradeapi.GetMinAmount()

Signed-off-by: Victor Shcherbinin <8344eb9903b855e542dbc983faef8b3203318f41@gmail.com>

victorshch/pytrader