Allow for QAIC-based model selection in ds()

[dill]

per #132

@lenthomas: "Yes, I think this is an excellent idea for a feature enhancement. One could imagine fitting all the models up to max_adjustments (so that the fitted values of one are used as the start values for the next, except for the new parameter), and then calculating c_hat1 using the most complex one, and then doing model selection. It'd be good if it stored the c_hat1 somewhere as well (perhaps an attribute) and adds that to the printed output from the print and summary methods."