Easier customization of JSON encoding.

[carljm]

I have cases where I want to be able to pass some custom objects (which are not JSON serializable by default) as properties of a mixpanel event, from various places in my code. The simplest and most flexible way to deal with this (such that it can automatically handle my object within a list, or standalone) is to create a JSONEncoder subclass that knows how to serialize my class, and then use that encoder class for the json.dumps call.

It's not currently too bad to do this - just requires subclassing EventTracker and overriding the _encode_params method. But this still requires duplicating some things I don't care about customizing - the base64-encoding, and the MIXPANEL_DATA_VARIABLE.

It would be nice if either there were a json_encoder_class class attribute on EventTracker, or a more fine-grained _params_to_json method. (Names up for debate).