Refresh before request if neccesary and multipart only when resource and media body specified

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).
Isn't it possible to simplify the block?

~~~ js
if (!credentials.access_token && !credentials.refresh_token) {
  callback(new Error('No access or refresh token is set.'), null);
  return;
}

var shouldRefresh = credentials.refresh_token && isTokenExpired;
~~~