Change oauth client's site after successful login

Square uses `squareup.com` for authentication, but uses `connect.squareup.com` for their APIs. This PR changes the client's site once authentication has been performed. Otherwise, requests performed with the access token (ex. `access_token.get('/v1/me')`) are sent to squareup.com leaving you with a 404.

Making `:connect_site` a client option allows user to override the URL for staging sites in their omniauth config.