Expose execution price

Expose the execution price from the matching engine so that the trading
system does not need to maintain this information.
Expose remaining quantity

Expose the remaining quantity from the matching engine so that the
trading system does not need to maintain this information.
Make order cancellation idempotent

Align order cancellation with the order entry protocol by making it take
a new order size instead of the canceled quantity.
Remove order deletion

Now that order cancellation takes a new order size and exposes the
remaining quantity, remove order deletion as a redundant operation.
Use long integers for quantity in matching engine

Align the matching engine with the network protocols that use long
integers for quantity.
Merge pull request #8 from jvirtanen/fixes/matching-engine-api

Improve matching engine interface