[Python] standardize serialization, json, etc. naming

[suntzu86]

I see things like:

historical_data.json_payload()
covariance.get_json_serializable_info()
domain.get_json_serializable_info()
_build_gp_historical_info
_build_gp_historical_info_from_points_sampled

There's no reason for these to all be named differently when they all do the same task on their respective objects.