(For Fred) tweaks in support of partners

@fredsadaghiani I've added an override api_key arg to all the methods. However, because there are existing args with default values, to pass an override api_key requires that a client also supply all the earlier defaulted args. For the convenience of partners, I have also added methods track_with_key, score_with_key, and label_with_key, all of which take an api_key as the last arg.
Wow, I didn't know you could reference an instance variable as default parameter. That's really interesting..