Orders must be serviceable

[zyzek]

At the moment, agents can place as many orders as they like. They should only be able to place orders if they have sufficient assets to service them. Then aggregate demand and supply will reflect true asset levels, not be a distortion based on order book manipulation. What's on the books should all be potentially filled at any time.

[0xdomrom]

This was covered in #30 with available/unavailable_currency