Add dataset-download-options endpoint

[aazuspan]

This PR would close #64 by adding an api.dataset_download_options function to access the dataset-download-options endpoint.

It also adds the necessary payload and unit tests.

Let me know if you see and changes needed, @kapadia :)

P.S. I didn't implement the sceneFilter parameter that the endpoint accepts because it would complicate the implementation and, to be honest, I don't understand the point of it. Seems to me that if someone wants to know product availability for specific filters they'll be using those filters to find scenes and checking products for those scenes with the download-options endpoint. But maybe there's an application I'm not thinking of. Let me know if you think that's important and I can add it.

[kapadia]

Looks great, thanks @aazuspan! I take the point on scene filter, those can be complicated.

[kapadia]

A new package with your contributions are on PyPI.

[aazuspan]

Thanks @kapadia!