Implement the getting a cookbook version via API.

Create an API endpoint to hit to get a specific cookbook version. If that cookbook
or cookbook version are not found, a 404 is returned.

Create the CookbookVersion model and expand on getting a Cookbook version
through the API. A CookbookVersion is an artifactual record of a specific
version of a Cookbook.
Merge pull request #154 from opscode/show-single-cookbook-version-super-40

Support GET /api/v1/cookbooks/:cookbook/versions/:version