Export Posts using User ID rather than Username

Facebook page names & usernames can be changed via their respective services so the export option that's provided should use the user id rather than username.

I had a FB page that had changed its name & the exporter was only providing partial results. This fixes that issue for all of the included network plugins.

This makes it so the exporter checks for the user id & uses that if provided (still allows you to use username as a fallback [not sure if that's really necessary though]), and updates all of the export links so that they include the user id as a $_GET variable.

I don't have a Foursquare or G+ account set up to test those, but it's simple enough that I'd imagine they would work just as well as Twitter & Facebook do.

Thanks!