updated bSocial's twitter, linkedin and facebook connections

this implementation uses PHP OAuth (http://oauth.googlecode.com/svn/code/) and the Facebook client library (git://github.com/facebook/facebook-php-sdk.git)

the classes will probably change some more. i'm not 100% in love with how the classes are organized currently but we'll probably have to change them quite a bit depending on how they'll be used, but the basic building blocks are working.

I can think of at least two more todos:
* implement our own child class to base_facebook.php to implement our own auth key persistent mechanism. one place we might want to stash the facebook user's auth key is in user_meta, but maybe there's a better place.
* we still need to work out a mechanism to redirect an admin or a user to a fb login page.

See https://github.com/GigaOM/gigaom/issues/3547, https://github.com/GigaOM/gigaom/issues/3550, https://github.com/GigaOM/gigaom/issues/3548, and https://github.com/GigaOM/gigaom/issues/3551, https://github.com/GigaOM/gigaom/issues/3640

Edit by @misterbisson: additional discussion about items in this PR happened in a sub-PR in https://github.com/wluo/bsocial/pull/1