Currently when we make an API request and the request responds with an authentication error, we attempt to refresh the access_token using the refresh_token and then re-attempt the request. If the request has a multipart body, it will not be attached to the re-attempted request due to the constraints of the library. This is exceptionally true for multipart bodies whose data is coming from a readable stream. To mitigate these issues, this PR makes the following changes:

• Check expiry_date (an added property) of the credentials object to ensure that the token has not expired. If there is no expiry_date value, assume it has not expired. If it has expired, refresh the token before making the request.
• If there is no access_token in the credentials, and there is a refresh_token, use it to refresh the token before making the request.
• If there is no refresh_token, never attempt to refresh (because you can't)
• If you have no access_token nor refresh_token, return an error in the callback.

This will give more responsibility to the developer to ensure that their credentials are kept up to date.

Also added oauth2client.revokeCredentials() which will clear the credentials object and attempt to revoke the access_token.

Bonus: Affects #229 where multipart request was built for all media requests. Now multipart will only be built for requests with resource and media.body. If resource is only specified, it's a simple JSON request, if only media.body is specified, then uploadType=media is used with the body (not a multipart).