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).

As discussed IRL I'm going to kill `reduceNamespaces` I don't need as `createLookups` is already doing the right thing if I use a reduced dictionary. I'll also fix `allNamespaceSizes`.