Rake doc generation task

Adding a rake task so that documentation can be generated from the command line with `bundle exec rake yard`. This helps being able to do documentation generation in a portable way since all the options that would be applied are stored in the Rakefile itself.

I've generated the docs once again using the rake task, and here is a summary of the differences:
*  fix naming of archive properties (`sessionId` -> `session_id`, `partnerId` -> `partner_id`, `createdAt` -> `created_at`)
*  add `to_json` description
*  update version of Yard (0.8.7.3)
*  fancy quotes
*  changed `<tt>` tags to `<code>` tags
*  escaping of apostrophes used in HTML
*  charset US-ASCII to UTF-8