Added single Order/PriceNode processing to OrderBook

1. Added additional fields to Order type
2. Added custom constructor to Order type
3. Refactored Order type’s unit tests to reflect changes
4. Added unit testing helper function for generating orders
5. Added buyDiff and sellDiff fields to PriceNode Type and refactor
constructor to reflect changes
6. Added  updatePriceNode to handle Order processing
  - returns the resulting new Order msg
  - modifies the priceNode’s buy/sell diff
7. Added unit tests for the above function