Customer.retrieve should not need a refresh_token as a parameter

[rebelvc]

No description provided.

[coveralls]

Coverage decreased (-0.03%) to 85.62% when pulling a36a4c5 on rebelvc:master into aa83930 on milesmatthias:master.

[milesmatthias]

Thanks @rebelvc. I think we actually need this.

Synapse will give an "Error in OAuth20Authentication" error if the access token has expired, at which point they expect you to use the refresh_token to get a new access_token and refresh_token: http://synapsepay.readme.io/v1.0/docs/authentication-refresh

One of the things that's on the todo list is to automatically refresh the token and try again when our code encounters the OAuth error. Without passing the refresh token, we wouldn't be able to automatically do that.

If you'd like to work on that and send a PR, that would be awesome!