Allowing input filters to parse the incoming content of the facebook stream

Hey,

This is my first take on an activitystream submodule. I found that the generated nodes from the facebook module did not really generated what I expected from it. Also I had no automated way to control what is happening to the data.

That's why I wrote an integration with input filters and I think it could even be nice to have for more of these submodules.
Please review this patch and commit it if somoebody tested it with some other facebook RSS feeds