[WIP] new Authorization Preview API changes

Fixes #617

 - [x] update [List your authorizations](https://developer.github.com/v3/oauth_authorizations/#list-your-authorizations) to obsolete `token`
 - [x] update [Get a single authorization](https://developer.github.com/v3/oauth_authorizations/#get-a-single-authorization) to obsolete `token`
 - [x] switch to new endpoint if `fingerprint` provided for "Get-or-create an authorization"
 - [ ] drop shared response model returned from [Get-or-create an authorization](https://developer.github.com/v3/oauth_authorizations/#get-or-create-an-authorization-for-a-specific-app) to return `token`
 - [ ] update [Update an existing authorization](https://developer.github.com/v3/oauth_authorizations/#update-an-existing-authorization) to obsolete `token`
 - [ ] integration tests

cc @joshvera