Trading engine overhaul

Thanks to a major bug report from xaustinx, the simExchange trading engine has been overhauled. Problems regarding how the trading engine handled edited orders has now been fixed. Trade execution should now be faster. Problems where matching orders in the Order Book would not fill have now been fixed. Individual orders are now limited to a maximum of 10,000 shares in size. Orders in the Order Book will now be checked for margin when matched. Please report any irregular or strange behavior you experience while trading using the Ask a Question form in the Help section.