Changed Table Level Constraints and the Procedures

Changed the table level constraints on Price in the BUY_OFFERS and
SELL_OFFERS table to be >0 or NULL. Also updated the procedures. They
now also check to see if the second participant is also a client with an
outstanding offering, and if they are, it updates their offering and
holding as well. If not, then it is assumed that either they aren't a
client, or the client purchased the shares through someone else, so no
holdings are updated for them.
Also, when updating offers, they check to make sure the update is valid
and deletes any offerings that hit zero. This makes them compatible with
the triggers that delete offerings that hit zero as well so that they
can continue to do so when the tables are updated directly.