Ipfs cli tests

Basic `ipfs-cli` tests. Currently covers all the commands in the readme. There is a fair amount of repetition in the test file right now. 

I've seen this repetition before with CLI tests with `isaacs/node-tap` that I contribute to. IMHO the right solution is a generalized TAP test tool for CLIs like `sstephenson/bats`, but without having to use `bash` (ugghh!). I may end up making such a tool at some point, at which time I'll revisit these tests and DRY everything up.