Added ability to rename features

I'm not completely satisfied with this, but wanted to get it open sooner rather than later. I'll leave a few inline comments. The branch obviously won't build until we do something with `IvoryT` (I really wish you could re-target PRs in GH)

/cc @etorreborre @markhibberd @raronson 
Not that i know of. The problem is scoobi needs to send the `Seq` of `Facts` to the mapper, so it serializes it with xstream, which blows up because `Fact` has a thrift object in it and xstream and libthrift objects don't play nice.