Moved rb-tree into tree package

This also drags along the Order struct, which I would like to completely
hide away but can't yet. All tests pass, but have not been checked by
hand.

The trade package should be renamed as it now only contains OrderData
and Response. OrderData should be renamed to Order and Order should be
renamed to something like TreeOrder. This refactoring has only just
begun :)
Order -> OrderNode, OrderData -> Order

The Order struct is now the simple flat data structure representing an
order as it comes in off the network. The complex, adn frankly gnarly,
OrderNode is an order when it resides inside two red-black trees inside
the matching engine itself.

Very happy to get this done, was bugging my daily :)