Changed property selling logic.

The user stories give the option to allow the balance to go below zero.
I wanted to change that to not allow the purchase to go through.
Enhanced the PropertyManager.

Took the onus off of the Property class of looking at its group members
for ownership and moved it to the PropertyManager.
Clean up.

Changed the Owner public property of the Property class to be a getter
only.  The clients now will use the Sell() method.