Enable access_type of 'offline' and get access_token with refresh_token

1. Modified parameters of the OAuth2.getGoogleAuthorizeTokenURL. (see the code change)
1. Do not pass the redirect_uri if grant_type is 'refresh_token'

Hi.

I need an web application which shows MY calendar to other people.

But this module was not support the 'offline' access_type and grant_type of 'refresh_token' .

So I has modified some lines of code.

This code runs graceful in my case, but I can't sure.

If you don't mind, please check this code and apply changes.

Anyway, thanks for your code.