Allow saving and reusing access tokens

[SSheldon]

Access tokens last for a while (currently 180 days), but currently we don't expose any way to reuse an access token, so you must authorize a new one every time you wish to create a YLPClient.

A simple solution here could just be exposing the access token and expiry of a YLPClient and allowing users to create their own YLPClient from an access token, leaving storage up to them. A more comprehensive solution could involve storing the token in the keychain for users.