Prisoner's Dilemma refactor to meet new model standard format.

[thomasjohns]

Part of #223.

Notes:

• Right now the schedule type is set to RandomActivation via the "Random" argument in server.py. However, in the jupyter notebook example, the PD model behaves differently for different schedule types. model.py handles different schedule types, but there is currently no option in the front end visualization UI to change the schedule type.

[coveralls]

Coverage remained the same at 76.328% when pulling 300c2f5 on tojo13:pris_dilemma_refactor into 736c315 on projectmesa:master.

[coveralls]

Coverage remained the same at 76.328% when pulling 300c2f5 on tojo13:pris_dilemma_refactor into 736c315 on projectmesa:master.

[coveralls]

Coverage remained the same at 76.328% when pulling e069c08 on tojo13:pris_dilemma_refactor into 736c315 on projectmesa:master.

[jackiekazil]

LGTM