added block cmd

This implements the `block` command from #138.
Let's do:

```Go
  Short: "manipulate raw ipfs blocks"
  Long: `ipfs block - manipulate raw ipfs blocks

    ipfs block get <key>  - get and output block named by <key>
    ipfs block put        - save stdin as a block, returns <key>

ipfs block is a plumbing command used to manipulate raw ipfs blocks.
Reads from stdin or writes to stdout, and <key> is a base58 encoded
multihash.

`
```