Support ChangeOrderPrice and ChangeOrderVolume

[m1dnightc0der]

Currently, we have a single method to allow us to modify orders default String changeOrder(LimitOrder limitOrder) throws IOException There are times when you just want to modify the order price, or the volume, or both. When just modifying, if you pass in a new limit order, you might end up adjust the quality too as the it just so happens the volume you have does not match what the exchange has. By having the ability to just modify the price only, you ensure that if the order happened to be part filled, on the exchange, you are not increasing the quantity unexpectedly.

[m1dnightc0der]

okey, we can do this by just not serliasing null elements, i.e. pass in a null volume.