Add Facebook SDK

[aaronschachter]

Needed to measure installs and in-app conversions: https://developers.facebook.com/docs/app-ads/measuring/installs-and-in-app-conversions

[lkpttn]

I'm working on setting up that FB company so we can have it own the app rather than you or me.

[aaronschachter]

The basic integration tutorials instruct adding the Facebook App ID into the project Info.plist, like @lkpttn did in this stub commit.

I don't think this will fly if we want to keep our project open source, hopefully setLoggingOverrideAppID:(NSString *)appID; will allow us to pass an entry from our untracked keys.plist