Add licenses command to pod cli

I wanted a way to view the licenses used by pods in my projects. I was inspired by [this post](http://blog.human-friendly.com/are-you-breaching-open-source-licenses-accidentally) and [tldr;Legal](http://www.tldrlegal.com/). Let me know what you think.

Note: I haven't written any tests for this code but I can take a stab at it later.