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 
I guess it is hard because writing *Writables is pretty clumsy, but it would be nice if the datetime was one field and the attribute was the other. But I can live with it. Just an observation on the futility of non-compositional code.