Enable the usage of arrays of cookbook directories

I have a local directory of custom cookbooks and then use
Berkshelf to pull in other cookbooks into a vendor directory.
This change allows users to supply not only an array of cookbooks
but a mix including multiple cookbook directories as well.
Adding a test proving mixed arrays work as well
Merge pull request #34 from jesseadams/feature/allow-multiple-cookbook-paths

Enable the usage of arrays of cookbook directories