This is a partial implementation of eth_getProof (see issue #1349),
supporting only a request for the latest block and an empty list of
storage keys (i.e. Account proof only). I don't know if there's a better
way of implementing this, but this was what I could come up with.
Posting it here in case it's useful.
Example output:
```
> eth.getProof("0x67b1d87101671b127f5f8714789C7192f7ad340e", [], 'latest')
{
accountProof: ["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", "0xf8518080808080a0a00135c9ec2655cb6a47ab7ad27d6fc150d9cba8b3d4a702e879179116a68a60808080808080a02fb46956347985b9870156b5747712899d213b1636ad4fe553c63e33521d567a80808080", "0xf873a02056274a27dd7524955417c11ecd917251cc7c4c8310f4c7e4bd3c304d3d9a79b850f84e808a021e19e0c9bab2400000a056e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421a0c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470"],
address: "0x67b1d87101671b127f5f8714789c7192f7ad340e",
balance: "0x21e19e0c9bab2400000",
codeHash: "0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470",
nonce: "0x0",
storageHash: "0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421",
storageProof: []
}
> eth.getBlock('latest').stateRoot
"0x6a0673c691edfa4c4528323986bb43c579316f436ff6f8b4ac70854bbd95340b"
```
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