Main class of the EC2Broker this is a broker used to schedule tasks on EC2
instances such that the total cost is minimal while still satisfying the
given deadline constraints.
This class contains a number of EC2 constants that are used by the broker,
such as the string representations for the three pricing models, the
different instance types, the operating systems provided by tyhe EC2
instances, the geographical regions in which Amazon EC2 is operating, ...