Add Guides content

This PR introduces a couple of new guides regarding real-world interaction with the GitHub API. 

The first guide is all about authentication. We step through the [web application flow](http://developer.github.com/v3/oauth/#web-application-flow) in an extremely slow and basic sequence. There are two examples here: one that redirects a user to GitHub for authentication, stores the `access_token`, and makes requests with the `access_token`. The other uses Warden to handle some of the authentication process for the developer. Both of these are just Sinatra apps.

The second guide is about using Octokit to poll repos, cumulate data within them, and render information as graphs using d3.js. 

These samples also have correlating downloadable code: https://github.com/github/platform-samples/pull/1