todo:
+ support for properties
+ add constraints and(), not(), or()
+ build scripts in CVS (Scons?)

possibly:
+ constraint base class to allow non-derived constraints to default to an equality match against the parameter. Would allow more compact argument constraints for the common case of equality e.g. mock.foo('bar').will(...)
+ constrain allowable order of builder methods
+ pull out dispatcher functionality to allow different matching strategies (LIFO, FIFO)?

