Added 'pin ls' Command

This PR adds a `ipfs pin ls` command, where users can view a list of what is being pinned. It only shows direct pins (keys that have been added with `ipfs pin add`). Maybe we want it to show the indirect/recursive pins as well?
@mappum i think that's a design problem the commands lib has to handle. `-r` is a well established flag to mean recursive. clients need to be able to use it.