Added cookbook support

It looks like there is already one pending pull request to add cookbook support to pychef.  I've taken a slightly different approach by using _latest as the default version to load.  I've only performed light testing...

Thanks
Thanks for reviewing the pull request.  It's been awhile since I wrote this, but I recall writing this for use in generating a tabular cookbook report in which the column headers were environments and there was a single row per cookbook, with the fields mapping cookbook versions to each of the environments.  Working with several environments, it was a quick way to ascertain what the version differences were and to quickly identify a few version issues (e.g. catching not having tied the "production" environment to a specific version, potentially a bad thing if uploading a newer version of a cookbook...).  I might have used the versions method as part of that, but don't recall exactly if it was useful for my purposes or if I just threw it in...