Add POE reference implementation

The reference implementation is based on Java NIO API and uses Nassau
for NASDAQ SoupBinTCP 3.0 support:

    https://github.com/jvirtanen/nassau
Add trading system

The initial version of the trading system merely responds to Enter Order
with Order Accepted and to Cancel Order with Order Canceled.
Merge pull request #5 from jvirtanen/features/order-entry

Add POE reference implementation