Implementing linked list order book & Making LimitTrade.compareBuy and LimitTrade.compareSell Private

I changed the OrderBook.OrderBook ArrayList that holds the actual orderbook to a LinkedList. This was done because the list will be inserted to a lot and this is less computationally intense with a LinkedList rather than an ArrayList. The other reason is that since we won't be searching very deep through the LinkedList (other than to insert an element, done with an iterator) we don't need the quickness of ArrayLists.