Allow `knife cookbook site share` to omit category

This adds a feature for knife to lookup the category on an existing cookbook. This enables uploading new versions of existing cookbooks with much less pain.

It isn't clear to me if this should be against the `11-stable` or `master` branch. I'm not sure how that works, and `CONTRIBUTING.md` doesn't really say what branch you'd prefer to receive PRs against.

Fixes https://github.com/opscode/supermarket/issues/826 as well.