Pinning

Can we match the command line interface here: https://github.com/jbenet/go-ipfs/issues/172 
I want to make sure that commands fit right in the hierarchy (`ipfs pin` has `ipfs pin add` and `ipfs pin rm`). It's more typing, but this keeps the `ipfs <cmd>` surface area smaller, and makes perfect sense when moving to http: `POST /api/.../pin`, `DELETE /api/.../pin/<key>`, etc.