added block cmd

This implements the `block` command from #138.
```Go
  UsageLine: "put"
  Short: "store stdin as a block, outputs <key>"
  Long: `ipfs put - store stdin as a block, outputs <key>

ipfs block put is a plumbing command for storing raw ipfs blocks.
It reads from stding, and <key> is a base58 encoded multihash.

`
```