Allow oauth post requests and add add_shelf and add_book_to_shelf

I wanted to get this on your radar because I found it useful. I didn't write tests for the methods since they were rather simple, but if you like I can get some up in a few weeks.

Note that I did change the oauth_request signature since it was only used in one place internally. However it might break client code that actually uses that method, I'd like to get your thoughts on a better way of organizing that. (or maybe that it's okay as is).