Changed compareBuy() and compareSell() methods to private
Changed OrderBook ArrayList from an ArrayList to a LinkedList. This was
done because when inserting LimitTrades to the OrderBook they would
likely be at the beginning of the list and therefore it'd be more
computationally intense to insert them with an ArrayList than with a
LinkedList
Merge pull request #1 from colmex/ImplementingLinkedListOrderBook

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