Fix specs, add delegators, fix callback_url as param, update README

I noticed there is quite a mismatch between the documentation and the implementation of the current version of the library on the master branch, so I went about trying to correct that.

Along the way, I discovered (and fixed) an issue using callback_url, and some missing delegators.

I know I've kind of made some big changes to the README, so if you need me to fix or undo some things, please let me know.