Support Notifications in the Facebook adapters

The Notifications support in the Facebook OpenGraph API is still in
beta, so this is likely to change at some point in the future.  In
particular, the paging API currently doesn't seem to work at all,
perhaps due to the "until" parameter being incorrectly formatted by
the Facebook backend.
Merge pull request #114 from chriadam/master

Support Notifications in the Facebook adapters