added block cmd

This implements the `block` command from #138.
```Go
  UsageLine: "get <key>"
  Short: "get and output block named by <key>"
  Long: `ipfs get <key> - get and output block named by <key>

ipfs block get is a plumbing command for retreiving raw ipfs blocks.
It outputs to stdout, and <key> is a base58 encoded multihash.

`
```