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'm a little worried about the partition sizes - I really don't know the full consequences of what will happen after a rename. Also see `reduceNamespaces()` for the size adjusting.