Display which cookbooks are contingent upon a specific cookbook.

:fork_and_knife: 

- New tab on the cookbook show page
- API endpoint
- Note that this is only 1 level deep in the dependency graph

Closes https://github.com/opscode/supermarket/issues/765
For both of these cookbook helpers it'd be nice to see either these methods tested directly or test their output within the api spec comparing a the json output to a known signature. You could do something [similar to how we're testing](https://github.com/opscode/supermarket/blob/master/spec/api/tool_show_spec.rb#L28) things in other api specs.