Implement `b2_get_download_authorization` operation

[jaredreich]

Hi there @yakovkhalinsky! Thanks so much for your work on this lib!

What are the chances of adding a method for the b2_get_download_authorization operation? Looks like it's listed in the API docs, but not available in this lib.

API link: https://www.backblaze.com/b2/docs/b2_get_download_authorization.html

I just did a run-through of all the available operations, and this is the only one that's missing, as of 2017-11-14.

[yakovkhalinsky]

Thanks @jaredreich for your question.

I'm a bit short on time these days, but I'm happy to take pull requests and open the repo to more contributors 👍

[jaredreich]

@yakovkhalinsky PR: #37

[jaredreich]

Added in #37