Use the Google or Twitter picture on user creation

I've switched out passport-google (openid) with passport-google-oauth which allows you to retrieve profile information. One downside is that you now have to create a key on the [Google API Console](https://code.google.com/apis/console).

Rather than fetching/cropping the profile picture from Google's servers, I've added a `remote` content backend (similar to local/amazons3) which only implements the getDownloadLink method.

I've enabled this functionality for Twitter as well.