API Authentication

:fork_and_knife: This implements API authentication using [`Mixlib::Authentication`](https://github.com/opscode/mixlib-authentication). This also moves the cookbook un-sharing mechanics to the `CookbookUploadsController` which allows us to isolate authentication to only this controller. Also when you think about it you're essentially destroying a `CookbookUpload`.

Additionally this expands upon `ApiSpecHelpers` so the `share_cookbook` helper method, which is used to simulate cookbook sharing requests, isn't a huge mess of procedural code (just a little bit now :smile:).