Add JSON serialisation for Listen class

[Skivling]

I have been developing a script using this, and just been rate limited as I was downloading my listening history on every run... So instead I'd like to serialise and deserialise the json data. This should be a function in the Listen class.

[Skivling]

Upon further inspection of the code, it seems there are the two functions for going to and from the api payload schema, they just aren't easily accessible when using the library, and are not documented.