Add methods for creating models

[tkem]

As discussed in https://discuss.mopidy.com/t/unable-to-create-a-playlist-from-a-podcast-uri-in-mopidy-js/737, clients sometimes may want to create models by themselves, e.g. to add tracks to playlists. Currently, this is only possible by creating a plain JS object with the semi-documented __model__ property.
Although the use case discussed above concerns the playlist API, there may be other cases where creating models, especially Refs, may be desirable. So adding functionality to create these on the client in a way that's guaranteed to work with current and future versions of the API would be a welcome addition.

[jodal]

Hopefully this won't be necessary as we're designing the core API with the goal of taking URIs in and returning Refs out. The client should usually not need to pass any models back to Mopidy.

[tkem]

So, how would a client add a radio station with URI and name to a playlist?