Display which cookbooks are contingent upon a specific cookbook.

- New tab on the cookbook show page
- API endpoint
- Note that this is only 1 level deep in the dependency graph
add additional helper specs
satisfy rubocop
change contingents to return CookbookDependency so versions will be correct
remove the contingent header, include with dependencies
Merge pull request #774 from opscode/cookbooks-dependent

Display which cookbooks are contingent upon a specific cookbook.