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 
Could you pass `subdict` to `reduceNamespaces` instead of the full dictionary? It looks like `prepareGlobsFromLatestStore` will filter out any paths with namespaces not in `subdict` (unless i have missed something).