Add Authorization Support to API

This PR introduces two methods to Instagram's API, ```get_authorization_url()``` and ```authorize_user()```, both of which can integrate very easily into Instagram's server-side authentication flow. I needed this for an app I'm building, and I tested it with the app and it seems to work fine. I also wrote some tests for ```get_authorization_url()``` as well as detailed documentation about its use in the README. Unfortunately I couldn't figure out a way to cleanly test ```authorize_user()``` since it requires a live redirect URI, but as I said I tested it out with my app and it seemed to work well.