download user profile picture from graph api

[sarim]

trafficstars

fixes #25

[julianlam]

While I have no specific objections toward saving the image to disk, can we not simply link to the image directly off Facebook's servers?

Also we might want to link this into image upload hooks... might be easier than saving to disk directly.

[sarim]

Actually I don't have previous experience with nodebb, so i may not be able to follow correct programming convention here.

In my experience, the image links from facebook aren't permanent. They expire/same link doesn't work for every user etc...

About image upload hooks, it seems like User.uploadFromUrl isn't implemented.

        if (!plugins.hasListeners('filter:uploadImage')) {
            return callback(new Error('[[error:no-plugin]]'));
        }

It depends on external plugins to implement it, right? So sso plugin need to have the code for downloading the picture, right?