Support OAuth signing in private repositories

The API can't be fetched for a private repository without providing a valid access_token. So it can't be used to signing (otherwise signin can't be done for private repositories).

This PR:
- handles oauth informations returned by 401 and 403 errors.
- provides methods  `Prismic.oauth_initiate_url` and `Prismic.oauth_check_token` (which does _not_ depend from any API instance) to allow to sign-in to a repository.
- deprecates methods `Prismic::API#oauth_initiate_url` and `Prismic::API#oauth_check_token`.

This PR fixes #31