Facebook iOS SDK 3.8

Summary:
     see https://developers.facebook.com/ios/change-log-3.x/

    Upgrading from 3.7
     https://developers.facebook.com/docs/ios/upgrading-from-3.7-to-3.8/

Reviewed By: chrisp

Test Plan: Release Testing.
Merge Fb SDK 3.8, remote-tracking branch 'upstream/master'