Return results from all API methods

[DanielLarsenNZ]

PlayerApi.GetCurrentPlaybackInfo() returns null when no devices are connected. I propose we return a result which reflects the API response to make this less ambiguous, e.g.

SpotifyResponse
    HttpStatusCode = 204,
    Message = "No content",
    IsSuccessful = true

This could be a base class for the current response models, e.g.

CurrentPlaybackContext : SpotifyResponseModel
    SpotifyResponse
        HttpStatusCode = 204,
        Message = "No content",
        IsSuccessful = true
    Timestamp
    Device
    ...

[DanielLarsenNZ]

Related to #50