Incorporate feasibility constraints into trader behavior

[davidrpugh]

There are two feasibility constraints that constraint any behavioral rule...

1. Markets are incomplete: a TraderLike must not sell more shares than he owns (i.e., no shorting).
2. I need to make sure that traders bid/ask quantities respect the fact that there are a fixed number of shares of stock available for trading: a TraderLike must not issue a bid order whose quantity exceeds the difference numberSharesOutstanding - currentShares.